I've been using it for weeks without problems and now it has started crashing.
It happens when i start it, just before he mounts.

I have all addons disabled

TBC Server



 

As I remember, the SINGLE_STEP thing was related to framelock.
It's the fightclass doing it - but probably not solely at fault. If you post the full error log, maybe the developers of your fightclass can take a look what might be causing this