Hi Andreas,
I'm Sumit, here to answer your query at the Microsoft Community.
Apparently, you are not alone - I have also been unable to open Workspaces for a week or two.
Another customer was able to fix this using a Reset sync feature as mentioned here:
https://answers.microsoft.com/en-us/microsofted...
Go to Settings
Under the Profiles > Sync
Scroll down to the bottom, you should see "Still having sync problems? Try another option", click it
You should seeing this:
Click "Reset sync"
After the resetting, my issue was fixed.
Hope it helps anyone who has the same issue.