sansnil

Wow, that's very strange as it should work then.
Did you reboot after changing the registry setting and verify it's still at 1?

The only other thing I can thing to try is to delete both of your ActiveSync accounts from the device and then reboot.

Then first add back the account which you want to sync wirelessly.
Then sync once with the local computer - it shouldn't give you that error message but it won't have any options you can check to sync.
Once you've done the initial sync with your local computer, disconnect the device from activesync and modify the sync settings through the ActiveSync program on your phone.
Then if you resync with the local computer it should work.