Can you please let me know exactly what the error message regarding the compass says?
Also make sure the ROV has plenty of slack, it will tend to dip nose down if you are trying to dive or drive forward and the tether is not slack enough.
Ok, this is a known issue, and disabling the check is a fine fix. I am working on an update to resolve this and other things that have come up frequently since the last update.
The error is caused by the ROV moving (rocking on water) while it starts up.
I thought as much as the difference between the two occasions I have powered up the ROV in the last two days was the ROVs orientation when I inserted the battery.