Sorry for getting back this late, was too occupied with ill kids and work...
I have looked at the PURGE .bat file. Looking at the code tells me it cleans up potential former installations. This is not the case here, didn't mention this but the issue occurs on a brand new machine, no dangling left-overs nor roaming settings.
Also checked the multimonitor FAQ, especially the per-screen-resolution settings. It had already been on before running into this issue. Also note I do not create fences on my auxiliary screen, just on the primary (= laptop) screen. (I had already learned that using fences on changing auxiliary screens/beamers/... is not the best idea.)
Good news though, the issue no longer occurs, even though I didn't make any change to Fences. I suspect some driver update fixed the issue. I will further observe it and get back here in case the issue reappears.