You have ECX installed right.
Well the amount of all models loaded (character models, auras, map models,...) is more than 512. And thats more than HL can handle. So it throws you out.
You have 2 choices.
Either play a map with less ENV models (though the map doesnt really have that many to begin with it may just work if the number is close enough).
Or you remove characters from ECX so that their models dont get loaded. You can do so by removing the charactername.amxx files from the core foulder.