Hi @Callum Hawkins-Row,
Thank you for posting in this community.
Your question seems to be related to the co-editor. We need to narrow down the problem and try some things.
1.First of all, it is very likely that your co-authoring feature is blocked.
You can refer to this article (Troubleshoot co-authoring in Office) to confirm if some of your actions are blocking the co-authoring feature.
2.Please try to update your office to the latest version.
Firstly, to avoid data loss, I recommend you click Save a Copy to save your change. Then update Office via Open a spreadsheet > file > account > update options > update now. Then edit in the file and see if issue still happens.
3.Please check if your other users are experiencing the same problem.
Please try using another user's computer and account to see if the problem persists.
4.Please check if there are problems with all the excel files.
Please try creating a new excel file and opening it the same way to see if the problem persists.
5.Delete the Cache in Office Upload Center and check whether you can save or not.
Click on Windows key> Enter Office Upload Center> Click on it to Open> Settings> Delete cached file> Ok.
6.Please make sure that SharePoint's co-editing feature is working properly.
Please try creating the excel file directly on SharePoint (not sure if you are using on-prem or online) and then see if the problem persists when co-editing.
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