Offline files don’t sync at logoff over WLAN

After sccesfully using offline files on several computers in XP Pro since years, I now have a laptop that won’t automatically sync at logoff when connected over WLAN. When connected via wired LAN it works, but when connected via WLAN the first thing that happens at logoff or shutdown is a loss off the wireless connection, so syncing is no longer possible.
How can I force the wireless connection not to drop first ?