My x51 stopped syncing on the 5th August 2007, so I waited a few days until it bothered me then on 8th August (after it didn't fix itself I started investigating). Note: Using my wireless connection I could sync most things that I used daily (RSS feeds, journal bar), but not others that talk only via active sync (eg. Tiny Stocks). I tried the following things until it worked again:
- Soft reset Axim - no luck

- Uninstall AS4.5 and reinstall on PC - no change

- Install Active sync 4.5 on my other PC (which had a fresh install of XP on it) - Still no sync, so this eliminated active sync

- Cleaned contacts on axim and cradle (the cradle was quite dusty) with paint brush and metholated spirits - still no sync

- Removed battery from axim and refit - no change

- Uninstalled VPN driver in device manager (as it had an X beside it) - no change

- Soft reset Axim again - now syncing after going thru partnership setup again. :approve:
So something in the last few steps fixed it, but I couldn't say what.
I should add my battery is as old as my Axim (2+ years) and only lasts a couple of hours watching a movie (instead of 4 hours), so maybe the battery has something to do with it as noted in an earlier post.
I'm not confident it's finally fixed, but I'll post again if it stops working.