@Rockeem Anderson
Does this problem also occure with other apps of Office online server?
Which browser does this user use?
Please keep the web browser up to date.
You may add the URL of Outlook Web App on IE as one Trusted Site.
Besides, please go to IE browser > Tools > Internet options > Security > Trusted Sites > Custom Level, make sure the option of "Include local directory path when uploading files to a server" is enabled.
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