Hi @Mark Whittaker ,
According to your description, it seems that this problem does not occur in Windows systems, so this should be a problem with the compatibility between the Mac environment and the software. I don't have a Mac environment to test, and I can't determine the cause of the problem. So let's do some troubleshooting:
Were there any updates in between? If so, did the problem occur after the update?
Try to use a different browser or your browser’s InPrivate mode to access SharePoint Online. See if the problem persists.
Check whether all document libraries have this situation.
If circumstances warrant, you can disable check-in and check-out.
Hope this helps.
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