ok, solved it myself by tweaking the registry.
HKEY_LOCAL_MACHINE/SOFTWARE/Research In Motion/BBAttachServer/BBAttachBESExtension/BBAttachServerName was not set to 127.0.0.1 but to another IP adress instead. After changing the value to 127.0.0.1 and restarting the BES, everything works fine now.
No idea, how the wrong value came in there, but who cares
