Hello Dan,
I'm sorry to hear that you've been experiencing this issue with SharePoint Online libraries. This error message usually occurs when a file is being edited by another user or when the file is open in another program.
I checked the troubleshooting information you provided. To my knowledge, you have checked all the possible troubleshooting steps, I'm afraid this issue needs to be investigated from the backend side via remote assistance and collect the background logs.
As this Forum community has limited resources, the backend support team there has more permission than us, they have more resources to investigate the issue from the backend, they have Escalate channel to escalate your issue to check more and collect log reports for the root cause.
In this situation, we sincerely recommend that your Microsoft 365 admin, can follow the steps in this article and create a support ticket to contact Microsoft online support.
Note: To create a support ticket, you need to have Microsoft 365 admin privileges.
Reference: How do I find my Microsoft 365 admin? - Microsoft Support
Please accept my apologies for redirecting you to the related team support, as the moderators in this community have limited resources to verify the backend information and to get the quick and better assistance, we requested for this.
Your understanding and patience are greatly appreciated. Have a great day!!
Sincerely,
Tina | Microsoft Community Moderator