Hi Butlin,
Delete the Cache in Office Upload Center and check whether you can save or not.
https://support.office.com/en-us/article/Delete-your-Office-Document-Cache-b1d3765e-d71b-4bb8-99ca-acd22c42995d
Does the issue occurs only with specific file?
If YES, I suggest you copy the data in the problematic Excel file and paste it in new Excel workbook for working and check whether it works.
Best Regards,
Karthick Rajan