I know that this has been an issue with Nuendo since the early stone-age (or at least it seems like that). Is there still no proper solution for canceling an Offline Mix Export immediately? 
It seems to take as long as the - actually canceled - Export to get the application back to normal.
… this just happened under Windows 7 (had to wait for almost three minutes to cancel the Export of a four-minutes Project), but I’ve seen this in pretty much every incarnation of this program, under any valid OS.