I have this same problem. As soon as I remove Avatar from the columns, I can no longer flag an email by clicking the flag symbol.
Would be great if this bug could be resolved in a future update.
eM Client would be already aware of this flag clicking problem in V9.2.1222 due to “many users having this issue in this thread”, and hopefully soon will have a fix.
In the meantime (if Set Defaults in Columns Configuration doesn’t work) for you as @Gary advised above to try, then just enable the “Avatas” in the Columns configuration. This will then allow you to select / de-select the flag for now. I’ve done that in the interim and it then works fine with flag clicking.
Note:- If you do enable the Avatars “as a temporary work around” and you don’t like the default large size Avatar, then go to “Menu / Settings / Mail / Read” and select “Small” next to “Avatar size” near the bottom. At least then they won’t be very obtrusive. I run my Avatas next to messages in Small size.