There is another odd thing I noticed that might be relevant: After the internet has dropped out, I can disconnect from the wlan via the icon but I can't reconnect.
I would be very happy if you could help me with this problem. If you need any other information, tell me and I will post it.
EDIT: After some more searching, I found some posts/articles that describe exactly the same problem as I have:
Unfortunately, in some posts the problem stays unsolved and in some others, where solutions are posted, the solutions don't work for me, so I really count on your help.