after 6 days of try and error and desesperation with this problem, my findings are the following:
to upload a simple blink sketch to my Ardunio UNO, with nothing connected (blank board), I need to go back to Arduino Version 1.8.5 (from 2017, if I got this right). This is the only IDE Version which is working wih my 2 Windows 10 PCs. And even more strange, in the IDE settings, I need to choose Arduion Nano as board. No version above this (from 1.8.6 upwards) is willing to do an upload. Ok, it works, and it works now also with my Arduino on a breadboard Version (with 1.8.5). However, still not the slightest clue what the problem could be – regarding the fact, that before, the actual Version 1.8.14 was working well for months ……