tigre i can help ok what you want to do is
-start ET
-open the console (before you go into a server) (use the ~ button)
-(in the console) type /com hunkmegs [then the number...should b about 2/3 of ur RAM]
ill show u and example /com hunkmegs 200
-then close ET
-Start ET again
-open the console again
-Type /com hunkmegs
-If it says the number you entered you did it right
This happens because you dont have enough memory to load a map. By increasing your hunkmegs you will be able to load maps. (Be sure to not go over 512 when entering the number) I use 200. 150 should be fine to load any map.