Where are the fonts being moved from and to? The standard fonts folder or other locations?
If you are moving into the system fonts folder then that is wrong as you should use Copy INSTALLFONT which will register the fonts with Windows.
Moving fonts out of the system fonts folder could also cause problems since they should be unregistered before being deleted.
Neither should cause a crash though...
Does the same crash happen if you perform the same operation in Explorer instead of Opus?
If the standard fonts folder is the source or destination, how is it displayed in Opus? Like all other normal folders in Opus, or like a folder in Explorer and the My Computer folder in Opus? (This depends on your "treat folders as virtual" settings in Opus.)
Depending on the view mode and/or columns shown (plus other things like shell extensions, e.g. ones which display overlaid images on file icons and are potentially called for all files), it could be that something is crashing when it tries to extract information from a half-written font file. The FAQ on general slowdown or instability investigation steps contains various ideas that may help narrow down the cause of the problem.
Also see if it only happens with certain font files or certain types of font files. If it does, zip some up for other people to test moving the same files between the same types of folders. (If it seems to happen with any font files then there's no need to create a zip, of course.) If the problem happens for everyone then it should be easy to pinpoint and fix. If it only happens on your machine then it's probably due to a 3rd party component which can make things trickier.