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?