I also get errors constantly, though it's usually #N/A, not #DIV/0. Things that can trigger 'em:
* Mistyping an item or enchant. Go to cloak enchant, type "asd", hit enter. Boom! Now go fix the cloak enchant. It doesn't help, everything is permanently broken.
* Pulling from an armory with an item or enchant the spreadsheet doesn't recognize, such as the Flexweave Underlay I currently have on my cloak. "Zorba/Shattered Halls" if you want to try it out.
* Deleting the contents of a gem. Again, this throws the system into some kind of infinite error state.
I've uploaded a "before" and "after"
here. All you have to do to turn "before" into "after" is write "asdf" in the cloak enchant field, then hit "cancel" when it complains. I've tried this both with macros enabled and disabled (since it starts out disabled and I always forget to re-enable it.)
If you go into the "before" sheet, open up Arms, and load Zorba/Shattered Halls, pressing OK when it complains about my cloak enchant, you'll get a #DIV/0 error.