Found: synchronization
PDA Sync Problems and Tips
Article: After several days, weeks, months of successful syncs one day your pda just will not sync. You remove activesync, and reinstall and still no sync. You check device manager, remove the com port driver reboot reinstall and still no sync. This is actually a common problem, but it's not documented. Windows by default will increment the com port assignment each time it detects a new device. It does so automatically via plug and play interface. So if you see your outgoing or incoming port assignments in device manager listed with any number above 8, then your pda will not sync. Most ipaq's (and other windows mobile based pda's) sync with a windows pc but you must use a com port assignment below port 9. Also this might sound a little confusing at first, because most pda's are connected via the usb port. But note that the actual com port assignment is virtual and basically used as the interface. It has nothing to do on how your pda is physically connected. Meaning the tip here works if you are connected via serial, usb, bluetooth or even via wi-fi link.
Simply removing the com port doesn't work, because as already stated windows will increment the port number each time a new device is added. Which is in essense what you are doing when you remove a com port entry, and try to reinstall your pda.
The solution is simple, select any communications port number under port 9, and you will have no problems.
To change the com port assignment do this: Right click on my computer, properties, then right click over com port 9(or whatever new com port windows just assigned to your pda) and select properties, click on port settings tab, then advanced. On the bottom of this screen use the drop down menu to assign it a different number. Remember it must be under 9. Just as long no other device is using that number then you'll be fine.