Sounds like you need to reload the popups.ini file
1) Open the Script Editor (Alt + R)
2) Click the Pop-Ups tab
3) Select File - Load
4) locate the popups.ini file (it should be in the directory that comes up automatically)
5) Close the script editor
6) Restart mIRC.
6a) This last step may not be necessary, but it won't hurt to do it in any case.