Hi Marvix,
You can check with 'get socket' if there are old hanging socket connections from telnet/ssh/WebUI. Maybe you are running into an issue there.
You can best try ScreenOS 6.1r4 in stead of r3. According to the release notes for 6.1r4 the following issue was addressed:
281995 - In some scenarios, all access to the management, except through the console, might be lost.
Hope it helps.
Casper