When a user opens an existing .xlsx file that was created in Excel 2010 with Libre and they edit it and save it. Then when someone else tries to reopen it with excel, they get an error

“Excel found unreadable content in ‘filename.xlsx’. Do you want to recover the contents…” error.
Doing an Open/Repair with Excel does not fix the issue.
MS Office 2010
Windows 7

LO works best with its native format ODS. It can happen that you got errors when working in other file formats.

Best thing to do: Save .xlsx as .ods, edit in .ods format and when done “Save as” .xlsx. This should work without errors in Excel.