1. Unfortunately in most of the tries, I cannot add any displays (e.g Point clouds) via the user interface, the selection box for selecting the display type is simply empty. I could not yet figure out when and why the box will be empty or not.

What happens if you go to the Plugins menu, and load the builtin plugin?

2. after quitting rviz via the menu or ctrl-C it will crash on the next start until I delete the ~/.rviz AND */.ros directories.

Can you get a stack trace?

Josh