After changing my main username after this tips, my Lubuntu laptop is only starting showing me only a dialog, that sais, it is in low-graphics mode with only one button "OK" and then a menu opens up with some options, but there I cannot select other than the first option:
What would you like to do?
[x] (Try running with default graphical mode)
[ ] (Reconfigure graphics)
[ ] (Troubles boot the error)
[ ] (Exit to consol login)
([cancel] [ok])
I can reach cancel and "ok" with the TAB key, but I tried all combinations of TAB, arrow keys, enter, space and pg up and down to select another option than the first, but nothing worked. Also attaching a USB mouse makes no difference.
How can I reach the option "Reconfigure Graphics" ?
Note: I am leaving this question open, although I've already recovered from this low-graphics-state (see my answer below) and there are many options How to fix "The system is running in low-graphics mode" error?. It still would be interesting, how to change the selected option in that dialogue, when it seems not possible.