robot
2022.10
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Robot Admin Guide

Last updated Feb 6, 2026

Execution troubleshooting

Message Too Large to Process

Observed Behavior

Processes using large amount of data (e.g. Excel files with more than ~2500 rows) can fail with the following error:

Message: Could not retrieve the result of the job execution. This might be because a message was too large to process.
Exception Type: System.ExceptionMessage: Could not retrieve the result of the job execution. This might be because a message was too large to process.
Exception Type: System.Exception

Cause

The output of the job execution exceeds the maximum configured value of the maxMessageSizeInMegabytes parameter in the uipath.config file.

This parameter has a default value of 10 MB.

To access the uipath.config file, go to the installation folder of the UiPath Components. The location depends on the deployment type:
  • When the Robot is installed in User Mode with the .exe installer, the uipath.config is found in the User Profile folder C:\Users\{{user}}\AppData\Local\UiPath\uipath.config
  • When the Robot is installed in Service Mode, the uipath.config is found in the installation folder. If the Robot was installed using the default path, the uipath.config file is found in C:\Program Files\UiPath\Studio\uipath.config.

Solution

Increase the value of the maxMessageSizeInMegabytes parameter in the uipath.config file to a greater value such as 50 MB.

User Access Control Prompt and UI Automation Activities

Observed Behavior

Unattended processes using UIAutomation activities such as click, type into or take screenshot do not work.

Cause

A program such as Server Manager set to launch at start up which displays a Windows User Account Control prompt blocks the Robot from making any interaction with the UI. This is happening because the User Account Control prompt is opened in a separate windows desktop to which the Robot does not have access to.

Resolution

Manually or programmatically find the program that displays the UAC prompt and set it to not start automatically to avoid the UAC.

Robot Fails to Start Execution

Observed Behavior

In this scenario, the Robot fails to start execution of a process, and throws the following error:

Executor start process failed, reason System.Runtime.InteropServices.COMException: A specified logon session does not exist. It may already have been terminated.

Cause A

The Robot machine does not have enough available resources (such as CPU, RAM, or Disk Space), so the Robot Executor is unable to start a process.

Solution A

Check or perform the following:

  • The Robot machine has enough resources (such as CPU, RAM, or Disk Space).
  • The connection time using the mstsc command-line function. It needs to be greater than 60 seconds, otherwise the error is displayed.

Cause B

The logon session cannot be created at the moment. This can happen in the following situations:

  • Login to Console is set to true for HD Robots. In this case, the Robot Service attaches to the current Console Session. This is not recommended for HD Robots, because there can only be one active Console Session at a time.
  • Multiple RDP sessions on Standalone Windows versions (not Servers). Standalone Windows versions can only handle one RDP session at a time per machine, whereas Windows Server versions can handle multiple RDP sessions.

Solution B

In this case, you need to perform one of the following:

  • Set Login to Console to false. This means that the Robot Service initiates an RDP connection from the Robot machine onto itself and attaches to it. This is the recommended method for HD Robots.
  • Check the Windows version, it needs to be Windows Server. Read more about Setting up Windows Server for HD Robots.
  • Check if the Studio/Robot version you are using is greater than 2018.1.3, which fixed an issue caused by the KB4088876 (Windows 8.1 or Windows Server 2012 R2 Standard), KB4088875 (Windows Server 2008 R2 SP1, Windows 7 SP1), KB4088787 (Windows Server 2016, Windows 10 Version 1607), and KB4088776 (Windows 10 version 1709) Windows updates. Read more about Software Requirements.
  • Increase the UIPATH_SESSION_TIMEOUT system environment variable on your server. The default value is 60 seconds, which might be insufficient due to slow performance on some machines. Please note that the environment variable is set in Seconds and the Robot Service needs to be restarted after modifying this variable.
  • Check if your Remote Desktop License is active on the server. You can find out more on this page.
  • Check if the Robots are in the right groups. Local users need to be in the same Remote Desktop group.
  • Check if the username of your RDP connection to the Robot machine is not different than the configured one. To avoid this error, sign off all the RDP connections on the Robot machine.

Password Not Provided

Observed Behavior

Creating a Robot in Orchestrator without filling in the password field makes it unable to start process execution. Changes made to privileges on the Robot machine can also trigger this issue.

Starting a process from Orchestrator or from the UiPath Assistant displays the following error message:

Executor start process failed, reason System.UnauthorizedAccessException: Access is denied.

Cause A

The Robot was created without a password.

Solution A

Edit the Robot as explained here and make sure the following fields are properly filled in:

  • Domain\Username - The username used to connect to the machine on which the Robot is installed. If the user is under a domain, you are required to also specify it in a DOMAIN\UserName format. Use the Whoami command in the Command Prompt to easily find it.
  • Password - The machine’s Windows password. Not required for Attended Robots.

Cause B

The SCM-managed Robot Service is not running. Find out more about Robot deployment types.

Solution B

Make sure the Robot Service is running:

  1. Click the Windows Start button, then search for and open Services.msc. The Services window is displayed.
  2. Find the UiPath Robot service and double-click it. The UiPath Robot Properties panel is displayed.
  3. From the Log On tab, select the Local System account option.
  4. Click the Apply button and close the window to confirm the changes. This ensures the Robot Service is running and has all the privileges it needs for executing processes.

Cause C

The Robot machine no longer has administrator privileges.

Solution C

Log in to the Robot machine with the same set of credentials with which you created the Robot in Orchestrator. If a process fails to start from the UiPath Assistant, then you need to contact your network administrator to allow admin rights for that user account.

'System.Collections.IEnumerable' Cannot Be Serialized

Observed Behavior

When using a Data Table in the "Wait For Task and Resume" returns the following error: "Exception: Type 'System.Collections.IEnumerable' cannot be serialized."

Cause

DataRow is not serializable, so it is not able to serialize DataRows while persisting the workflow. This can also be seen if we create a DataRow variable and try to wait (persist) after that.

Solution

Execute “Create Form Task” activity and build a list of task object(output) while looping through DataRows(System.Data.DataRow).

After that, we can loop through the list of task object(output) using “Parallel For Each” or “For Each” and execute “Wait for Task and Resume”.

Increased Process Execution Duration

Observed Behavior

Process execution takes almost twice as long when Studio is installed from UiPathStudioSetup.exe (Community Edition), compared to UiPathStudio.msi (Enterprise Edition). The issue occurs on Studio 2021.10 and above.

Cause

Studio Community Edition is installed in %localappdata%\UiPath, which is not a trusted location by Windows Defender. This means that every time a log file is created, Windows Defender starts to scan it, which prolongs the process execution.
The scenario does not apply to Studio Enterprise Edition, because the installation folder (%ProgramFiles%\UiPath) is trusted by Windows Defender, and thus not scanned every time a log file is created during the execution of a process.

Solution

Adding the Studio Community Edition installation folder (%localappdata%\UiPath) on the Windows Defender exclusion list means log files created during the process execution are no longer scanned on the spot. To add a folder on the Windows Defender exclusion list, you need to perform the following steps:
  1. Click the Windows Start button, then search for Virus & threat protection and open it.
  2. Access the Virus & threat protection settings page.
  3. Access the Add or remove exclusions page from the Exclusions section.
  4. Click the Add an exclusion button, then choose Folder from the drop-down menu.
  5. Navigate to the %localappdata%\UiPath folder, then click the Select Folder button. Please note that Administrator rights are required to add a folder on the exclusion list.

Log files created during the process execution are no longer scanned on the spot by Windows Defender.



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