Search for Ms office and click on uninstall, If you want you can try repair also but mostly repairing the ms office won’t the fix the issue.
Method 1: Reinstall Microsoft Office Open Run command and type appwiz.cpl and hit enter.To narrow down this issue, try to remove all clients from this mailbox, and leave OWA only. It seems strange if the disappear message is irregularity. Every x number of emails disappears completely from Outlook.
