HobbyDev:
Well - I didn't expect that! But I changed back to 2.5.0 (not beta...) and it works! Thank you very much. It took me a while to get it changed and even the first "build" after the change didn't work and I got a lot of new error messages. But finally ...
Last question for now: If I click on "upload sketch" I get the question whether I want to build the project before it will be uploaed or not or never or always. If I do not click on yes my changes will have no effects. What for do I have to make this choice?
I don't know. Previous Sloeber version did rebuild the project even if you only changed the COM port. This was fixed, but maybe it was the motivation for the question. Set Yes for always.