Hi,
>>whereby a user of mine couldn't open
So this only affect this specific user? Did the use see any error message when he/she open a .htm attachment?
Since when did the use experience the issue? Has he or she ever made any changes to the system or Office installation?
First, try to launch Outlook in safe mode, the verify result again:
Press Win + R and type "outlook.exe /safe" in the blank box, then press Enter.
Regards,
Ethan Hua
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