Since update lr masks no longer work and lr crashes

I updated my lr today and now when I try to a mask it just crashes or wont work.  Have tried endless things to diagnose with adobe agents but nothing so far has worked.  It started working a bit just now and the agent said it was due to the catalogue being stored on dropbox.  Worked this way for years, anyway figured he must be right as the masks were working the lrcat in downloads.  Now it's stopped again.  Tried updating software, returning to older version of Lr, clearning preferences files, caches etc etc and nothing is working.  Any help would be much appreciated as I can't work properly without using masks and feeling a little helpless.  Thanks