Hi @Ivan Parshin ,
Thanks for your reply!
Based on my research, seems like it's related to a transient backend incident. If you are the administrator, you might be able to see the EX661478 in the Issue history of the Service Health Dashboard:
And based on my test on Outlook for web, i can see the replied icon successfully:
So please log in to Outlook for web and check if the icons work normally. If not, please wait patiently and pay attention to the status update of this issue.
Hope to help you!
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