When opening up an Excel file in SharePoint Online, that is pre-set to be read-only, there is an option in Excel to "Edit Anyways" as it's read-only and "To prevent accidental changes, the author has set this file to open as read-only".
When clicking this "Edit Anyways" button, the following error comes up:
"We're having trouble connecting to the server. If this keeps happening, contact your help desk."
This happens when opening the file in Excel Online, and Excel in-app.