Using an outdated UT can lead to a few issues here and there, but the thing that will cause crashes are outdated XML files. These would be the files within:
C:\Nexon\Mabinogi\data\db
Since the Vate update adds new skills, any time you try to do anything relating to the new skills will cause the game client to crash, because the data for it doesn't exist in your modified client. In particular, UT uses:
C:\Nexon\Mabinogi\data\db\skill\skillinfo.xml
Until Shaggy updates UT, it is advised to unpack mods and disable data folder reading, then delete this file, or even the entire \db folder if you experience other crashes. Outdated TXT file mods (like in the \code and \xml folders) will just cause things to be in Korean or missing a name entirely (where things appear like xml.itemdb.4234 or whatever), but they won't cause the client to crash. An outdated \db folder will cause crashes.