XF 2.2 Erroneous Conversation notification icons for many users


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.
The first step is to disable all third party add-ons, revert to a completely default style, and remove any custom JS and caching.

It's not something which has been reported previously so likely it's due to something third party.
Right just to say this is sorted now.
A massive thank you to M@rc for helping me initially to rule out any addons and style or custom issues, and then to AndyB who persisted to track down what was a historical DB corruption issue that had left some orphaned rows in the conversations tables. AndyB wrote a few addons which diagnosed the issue and then corrected it for me. Thank you so much.
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