Hi TimonYang, thanks for the suggestions. Unfortunately it didn't solve the issue. And , as far as I know, I never published this project with ClickOnce ..
But I solved it .. by creating a new clean.net 4.8 project, adding the existing class libraries and all existing the C# source code .. until it could be build and run as before. Then I used this 'clean' sln to port to .net5 with the upgrade-assistant. With only a few errors to fix, it could be build and run.
(The project code of the first try that failed must have been cluttered with a lot of leftover stuff from over the years development and updates)