Ok, here's an easy way to fix this. If you have any favorite servers from any mod, bust out a pen/pencil and paper and write down the IP addy's!! Once you've done that, open up your Halflife folder and find a file called Favsrvs.dat Delete this file (note this will delete all servers off any mod you got installed, which is why you wrote down the IP of you favorite server to put back in
)
Example of how to find :
c:\Sierra\Halflife\Favsrvs.dat
Then all you have to do is open up ESF again and presto!! Hit update and find ESF server's. Since this happens alot, you might end up doing this alot, which can be annoying if you keep having to put back your favorites in. Well, hope that helps