Make a backup of the file
Open the file
Click on ‘File‘ and ‘Inspect Workbook‘
In my case it found ‘Custom XML Data’
If you don’t know where this comes from and what it does, remove it
Save it – should work normally now
https://answers.microsoft.com/en-us/msoffice/forum/all/the-document-was-saved-successfully-but-excel/edb4f192-546b-4fa9-8c04-42e3f631046b