Hello!
I did a complete reinstall of mIRC and the problem persist. I also removed all nondefault scripts and some highlight settings i used to have.
It is the right-click menu for nicks in both the nicklist to the right and the nicks in the channel window as well. Also in status window.
Context menu for channels/networks to the left works.
Edit: However, context menu for channel names in the channell windows also works as intended. It seems only to be the nickname context menu (where you could do whois, DCC, etc)
Edit 2: Another interesting artifact is that the editbox at the bottom of the chat (the one you type stuff into) highlights when right clicking a nick on the right in the nicklist, but not when clicking nicks in the channel window.
Popups are not defined. All the tabs are empty.
Hope this helps.

And thanks for replying !