Same problem here, mostly this happens when I switch to windows while Maniaplanet is loading the map. Quite weird, this issue started to occur after one of the recent updates (think it was the Storm release update).
Strange indeed,
I wasn't able to reproduce it.
Could you try to reproduce it with the latest ManiaPlanet_Log.exe, and send me back the corresponding log ?
Maybe it's linked with the map: a copy of the map could also help then.
But I've the feeling it's quite random...
Thanks