Player: V10.011.011
Any level that has pre-placed blockers immediately triggers the Access Violation error when starting the level.
Will try and make a hotfix for this later tonight.
Interestingly, this does not happen when running the mass replay checker on such levels.
Found which commit introduced this bug (c884b15; the one to fix window ordering on levels containing preplaced lemmings). Still need to find out why.
Fixed, uploading the hotfixed update now.