I managed to do a scan this afternoon. What i did was deleting the old IP ranges, and creating new ones. Then i clicked "Scan all enabled ranges now".
I therefore believe, that my problem was caused by removing all the SSH credentials last week. Might this be a bug? I'll try to reproduce the error maybe tomorrow or next week.
We started the scan this afternoon at about 15:00 CET and it is now (18:22 CET) still running (scanning two full 255.255.0.0 networks). At least I think it's still running, because at the moment, the machine is at about 95% CPU usage by LansweeperService all the time. Strange thing is that there seems to be no Newtwork Traffic. Most of our network is empty at the moment, so I havent got any results since 15:30 CET, when it probably finished xxx.xxx.0.xxx and xxx.xxx.1.xxx. Is this normal behaviour?
I hope the scan will be finished tomorrow morning, then i will retry to scan with new, but smaller IP ranges, so the tests won't last that long.
@tkerr: Did you click on "Scan all enabled ranges now"? After clicking it you should see the LansweeperService in Windows Task Manager using a lot of CPU. Then it takes some time i guess.