Editing WIT in VS2019 for TFS2018 and cannot save (TF237090)

Question

I use the Process Template Editor from the marketplace (Microsoft DevLabs). I load a Work Item Type from Project and (with or without changing anything) I want to save it. Then I get the error message: Failed to save the 'xxx' Work Item Type to the server. TF237090: Nicht vorhanden oder Zugriff verweigert. The last sentence means in english something like not available or no permissions.

I access the server from my workstation. Both computers are in the same domain. In the AD I am only a normal user, but at the TFS I am in the group Team Foundation-Administrators. Inside the website I can modify everything normally. Can it be that this is a direct access to the database and the database has to be set to access external requests?

Or has someone another idea where to look?