Hi @Sapp, Nicholas
Does this issue occur with all Excel workbooks or Office files?
I suggest you open workbooks via Excel desktop application when you encounter this issue.
Please change the browser to open, clear browse cache and cookies for a check.
Besides, please sign into admin account in Health > Service health page: https://admin.cloud.microsoft/?#/servicehealth, check the status of SharePoint online, see if there is any information related to this issue.
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