Erroneous Conversation notification icons for many users

[Kwaka]

Hi,
I did search, but I couldn't find any question like mine. I am on the latest build 2.2.15. For some time now some users have been complaining that the Conversation icon is indicating that they have 1 message, and when they click on it they get an error. See image examples below. I hoped it would go away by moving to the latest upgrade... it hasn't.


and when you click on it:


How can I resolve this?
It's for multiple users.

Thanks for any help.
Erroneous Conversation notification icons for many users

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