Troubleshooting: WindowsMobile 6.5 Sync with WindowsMobileDeviceCenter

After upgrading the ROM of my smartphone to WindowsMobile 6.5 I experienced the problem that after the first successfull syn with Outlook all future syncs failed. Several HardResets, re-established sync-partnerships all resulted in the same problem.
Even the usual problem areas for synchronization (Bluetooth- instead of USB-Connection, needed Ports blocked by Firewall,...) were not the case. As I had actually given up and wanted to downgrade to WindowsMobile 6.1 and therefore did some research, I cam across a forum entry on Outlook cashing caused by the use of invalid characters. Then I narrowed the problem to the contacts-library - as I was able to sync all other categories (tasks, notes, calendar) several times.
A manual check of each contact revealed some phone numbers (which I had created via WorldCard Mobile automatically by photo), had used illegal characters:
+49 12/345678
0123 / 456789

Since I removed the '/' and established a new synchronization-partnership from scratch - it works. So far, even after two reboots - both by phone and PC.
If there should occur other problems I post an update here.