A while ago at a mid-sized site, I was confronted with some strange userenv errors in my GPO's. Being not that fresh :-}, it took me some time before I realized the only way to find what was happening, was enabling verbose debug logging. An hour later everything was running like it should, because I was able to pinpoint the fault in the generated logs and took the appropriate actions. This all sounds difficult for a novice or even a system engineer (NO, NOT YOU, DON'T HIT ME!). Luckily Microsoft as always created a white paper which guides you step by step in setting up a debugging log. You can find the paper here.