I now reinstalled UM 20.04 on my desktop and everything is working so far.
But what I noticed for the second time were following problems:
- Virtualbox: Not the program itself, but the machine window gets stuck (like pinned) on the monitor and can't be moved. I wonder if there are others who have the problem. I figured out that I have to press the right-Ctrl key, then a right-click on the upper window border brings up a right-click menu, and I can go to Move. If that is intended, then it's a bit annoying.
- After the installation, and opening Rhythmbox, it crashed. This happened only once, and since then it works. It's a minor issue.
- After a display card problem, I said to Wolfman that I'm going to reinstall the system. I ran the ISO from the DVD, and tested how "repair the system" works. In contrary to the first installation, I tried to use "Sign me in automatically after the start". And again, I had the problem that the password window didn't appear in Software and Updates. This was the second time I noticed that. As a result of this, I reinstalled from scratch once more and of course, I chose the setting, where I have to enter the password after the start. I don't have that problem in my just installed Virtualbox version of UM20.04.