What I recommend doing is the following....
Make a backup of your current monsters folder. Replace half of the monster files with the ones you used previously (lower XP), then reload the server. See if it crashes still. If it does, then replace the other half of the monster files with your previous ones as well.
This will help you locate which file(s) in particular are causing the problem. The crashing should stop now since you said your previous files were working fine, so then we can determine whether the monster file(s) causing problems are in the second half of the monster folder or the first half. Once you locate which half is causing the problem, reload the newer monster files (with higher exp) and then replace half of the half of monster files that were related to the crash. If the crashes still occurs, then we know it must be in the other half of the half (unless multiple files are causing this crash). You can either look at each file individually then that half or keep replacing files little by little until you figure out which one had been causing the issue.
If after replacing every file with your previous, lower exp monster files there is still crashing, then maybe there's something new in the higher EXP set of files that eventually causes a crash (I very much doubt this though since nothing really would call upon such a new file to open).
I dunno if I worded all that very well. Hope it helps.
