Other user is running a file serving script (OmenServer) which I believe uses 3x mIRC specific DLLs. Not sure whether this could cause this sort of corruption. As far as I know the DLLs are used to provide file searching functionality and do not have any functionality to work with DCC SEND.

I do know the DLL author (though he wrote the DLLs perhaps a decade ago), so I may be able to get some help with debugging this if we think it is a result of DLL causing this.