In Administration -> Online Users, there used to be a list of online users who we can "Send Message". Now suddenly the list disappear. How do I get it back?
Isn't it merely hidden by personalization?
I don't think so, coz it happened to all users, not just some.
I would review (and debug if needed) the logic showing the list. Unfortunately I don't remember anymore how it used to work in Axapta 3.0, therefore it's up to you.
This might occur due to the Error Code 0X80071A90 as it corrupts most of the files and affects the OS form many aspects. So if it is then trying to fix the error. Hopefully, it can solve your issue.