Sounds like the default aliases and/or pop-ups have gotten misplaced.
This has been reported on earlier versions of mIRC also.
The simple solution is to:
1) open your remotes (Alt + R)
2) click the Pop-Ups tab
3) Click File -> Load, then locate the popups.ini file from your mIRC installation directory.
3a) If you have gone with a default installation, the directory with the original popups should be c:\Program Files\mIRC\defaults\scripts
3b) Note: This is based upon my installation which was originally done as a default installation way back at version 6.0 for this computer.
4) After you load the popups.ini, you'll have to go through each of the sections listed in the View menu and load the same popups.ini file for each section.
5) If you have any custom popups, then you may have to re-create them unless you can find the popups.ini file that contains those custom popups.