Error Problems occurred while saving plan

Hello,
are you able to save changes to a plan using the WebUI ?
Have you ever been able to save plan changes ?
I would vote for a permission denied: check your role(s) in the Project Area and verify that these roles are allowed to perform the action you made in the plan.
Go to Project Area Editor, Team Configuration, switch view to "Show all actions and roles"
and go to Save Work Item (server) - modify the work item

Actually, the work item eventually was saved. but i still got the error.