Error code 0x80810005 at ActiveSync on Windows Mobile 6

Hi,
I use ActiveSync with my mobile phone and Axigen Mail Server (has a
licensed ActiveSync interface). Now I have the problem that the
synchronization sometimes fails with status code 0x80810005 after about
80 mails of 90. The transmitted data from the mail server looks fine but
Windows Mobile 6 doesn't seem to accept that.

I haven't found any information about this specific error code and like
to ask if somebody knows that problem or what that error code means.

Although I'd be happy about a way to test the ActiveSync connection
without my Windows Mobile phone to check where the problem is. The full
log of ActiveSync on the phone doesn't gelp much unfortunately.

Thanks,
Hilmar