Not sure if you're having the same problem, but with me, it was because it wouldn't load in '
EX mode'. I fixed it by changing the 'Network settings for debug' from Single Settings to Dual Settings. You can't RTE while your PS3 is in dual settings mode, but you can play CoD 5 with no trouble
just switch back to single settings when you want to RTE, it takes 5 seconds.
Hope this helped ^^