Based on the error message, it appears that the faulting module is ntdll.dll. This is a critical system file.
Have you tried running the System File Checker (SFC) utility to check for and repair any corrupted system files?
You can do this by opening a Command Prompt with administrative privileges and running the command: sfc /scannow
After the scan is complete, restart the computer and check if MSTSC works in the user session.
Additionally, have you checked if there are any specific Group Policy settings or restrictions applied to regular user accounts that might be affecting MSTSC? It's worth reviewing Group Policy settings to ensure that there are no restrictions or policies in place that could cause RDP issues for regular users.
Best regards,
Karlie Weng