Well unless someone else have a other option this is what I found on a simuler problem.
Looks like you've been playing in an evil server. Period. Some servers force you to download their custom paks, and looks like that one had a custom pak with the same name as an official map. That generates a problem when connecting to a server. All I can recommend is:
1 - Uninstall and make sure to delete everything related to ET, specially the maps/pak
2 - Reinstall the game and disable the option to automatically download packages on the options menu.
so unless someone have a better Idea, this might be the solution.
if you delete it I would save you're cfg and etkey.
and 2nd option is not needed I think, this guy who responded on the question did this to prevent automatic download.
and you want to have automatic download
and in case you need to download ET again here is the updated version 2.6b