The action when double-clicking on the nicklist etc is list in Options then stored in mirc.ini so it's possible that mirc.ini was damaged and rebuilt incorrectly. If you look at options and find the row labeled "mouse", there are 6 rows there for 6 different locations. The 6 default values from top to bottom are:
/lusers
/whois $$1
/channel
/query $$1
/whois $$1
/whois $$1
If any of those are blank, you can restore the default action by pasting the item from the above list.
