I have this problem too.
A difference is that the BES is new and so all BlackBerry users are new to this system.
It seems that only one user has this problem and, yes you are right, in the log there are "log on" errors.
The question is: what can I do now to complete my throubleshooting?
Any help is really appreciated.
p.s. I'm new to this forum but as far as I can see this is great.