Sync With OMA Failes with Support Code 80072EFD

Dear all,

maybe someone can help.
My Windows Mobile 5.0 AKU2 connect just fine with ACS 4.2 and also via
wireless.
As soon as I wish to use the gprs connection I receive the error 80072EFD
with information "Waiting for network".
The curious thing is I can access https://servername.domain.tld/oma and
/exchange without any problem via Pocket IE on the mobile device.
Also I see no logs on our firewall as well as in the W3SVC logs of the
exchange during sync. Any Idea?

Thanks a lot,

Alexander