
Now:

Action by Reference two or more times!! Not always, and not with all actions but SO MUCH times, and especialy with older and complex actions (makes in 5.0.7) and more specially when you superpose this referenced actions, or try to delete'em after you referenced... you can do Moho crashes easily if experimenting...
There is another relationated actions bugs that can makes Moho crash, like when you delete his first keyframes (those that are before the arrow); or if you Delete an action, UNDO an then try to insert by reference, or if you copy an action inside an action (this is weir but you can do it, if there is an action referenced in timeline and you copy timeline keyframes into an action) this must do Moho crash, but if not, you can try to delete the action and Moho sure crases, yes... and more things that I don't remember now... This never happened before, and makes impossible (for me at least) working with actions in 5.0.8 (and I use it for ALL), PITY!!

PD: I hope that this help you, and (PLEASE) hope too it can be mended soon...
R.e.M