can't save or Autosave document

Hi all, I'm finding that occasionally I can't save my Numbers document (that is actually quite a large file).

When I try to save, I get the following error message...

This message did appear more frequently but I split up some of the data into separate tables. It happens less frequently now but I was wondering if anyone would know the cause so that I can eliminate it completely...

I noticed there is a similar post relating to a fonts issue, but I don't believe this is connected as mine saves ok probably 90% of the time.

Cheers

Ric