Dear Christian,
Greetings! Thank you for posting in the Microsoft forum.
Is it possible that someone else edited this workbook at the same time?
When this message appears, your data is safe locally, but you can’t resume coauthoring until you resolve this error. To work around this:
- If you have any edits you don’t want to lose, select Save a Copy and save the file under a new name.
- Reopen the live document.
- Add back any changes that didn't make it to the server copy.
Note:You can also select all and copy, paste the document into a new window, and then selectDiscard Changes*. You can then add back any changes that didn't make it to the server copy.*
If the issue remains, please provide us with the following information for further troubleshooting:
- May I know if the workbook is shared with someone else?
- Where the workbook is stored, OneDrive for business or SharePoint online?
- Please provide the screenshot of Microsoft 365 Product Information. To get this screenshot, open Word or Excel on your system, click File>Account, capture the screenshot of all the information mentioned under Product Information.
If my understanding above is not consistent with yours, you can also post back and point that out.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Neha | Microsoft Community Moderator