Does the VPN keep a log?
One idea --
1. Turn off DNS Relay (known bug)
2. Set QOS Engine Automatic uplink speed test to manual and input an approximately correct speed (works around another known bug that complicates the first one)
3. Restart modem, wait for 60 seconds, restart VPN
If this helps, it's the DNS relay bug known in 1.21, probably trying to do reverse DNS lookups on its own host as part of its startup process. You can operate with it off permanently if you wish.