Just resolved this problem on my van.
Where fitters had cut into roof channel for pop top in rear nearside behind trim, they left extremely rough cut edges. The loom had rubbed against this and although seemed only a minor cut in wiring it caused the problem.
Incidentally when I fitted replacement LED tail lights they showed a fault after a short time which was never explained. Now there is no problem.
For anyone having issues with strange errors it may be worthwhile checking out the quality of your roof installation. To say I am hacked off with installers for such an expensive job is an understatement.