Did you resolve the issue?

I'm having the same problem. I've searched the web and the closest I've got to a solution is that conditional formatting must be limited to 2050 rows. This doesn't seem to adequately answer the issue as I have other spreadsheets with a lot more conditional formatting rows that 2050.

I found that it is saves ok if I save the file as an earlier version of Excel. Not sure why this seems to work.