According to the information you provided, there are many causes of the problem, which are not yet certain. But don't worry, let’s do some checks:
- Check if the issue is specific to a particular file or affects all files. If it affects all files, try opening them in the desktop version of Office instead of the online version.
- Check if the Office Online server is configured correctly and is running the latest updates.
- Check if the issue is related to the Office Online server or the SharePoint servers. If it is related to the Office Online server, try restarting the server or reinstalling Office Online. If it is related to the SharePoint servers, try restarting the servers or checking the SharePoint logs for any errors.
For your reference:
Some Office files open in read-only mode in SharePoint Server 2013
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