The next time that the crash happens, submit it through the crash reporter, and be sure to add your contact information (email address).
To make sure that the crash reporter comes up, choose Help > Adobe Crash Reporter. Make sure that “Always show this dialog” is selected (even though, of course, it’s not “this” dialog box that will get shown, but the actual crash reporter dialog box).
Your crash is somewhere in the mask code, perhaps because of mask feather. You say that you're using mocha.
Do you mean that you're using the mocha Shape plug-in to get the mask into After Effects?
I'll send the crash report to Adobe today.
I used Mocha for the track data not for the mask as I felt I could get a better mask myself within AE. Mask is on a solid and footage is alpha matted through.
I seem to have fixed it by turning my mask expansion down to 0. Turning to anything else causes me to crash on the same frame as before.