Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I have had this issue recently and it turned out to be a company logo which was in the top right of each worksheet.I removed this (.png) image from each sheet, the workbook did not then give an error saving. I then opened the logo and re-saved it as a .jpg and then re-added it to each sheet. The workbook continued to save quite happily.
Hope this helps