There is a known bug in the Watchguard firmware that is causing a lot of trouble. Currently, we still do not know why VPN Tracker is triggering this bug, as the Windows client seems to not trigger it. We talked with Watchguard, but even they could not tell us what we are doing wrong. Basically the tunnel dies internally in the Watchguard (it's still shown as open and established, but traffic arriving over this tunnel is rejected as if the tunnel was closed).
If you are using a Branch Office VPN at the moment, try switching to a MUVPN if possible. MUVPN works with VPN Tracker as well and usually yields for better results.
Please refer to the following description.