Hi,
if you have some issues or feedback about Plastic SCM / Unity version Control please open a ticket with us or use the Unity forums: https://forum.unity.com/forums/unity-version-control.605/
I can see you are using the legacy GUI for your initial feedback. This GUI was deprecated and removed from the installer. We need to review what of these errors are happening in the new GUI and fix them (not necessarily to use the very last Plastic version but at least the new GUI).
Are you using AD authentication? If you open a ticket with us, we can propose a workaround to mitigate this error.
Please open a ticket with us so we can debug the logs and fix this problem.
Regards,
Carlos.