Everyone saying that using Installous is killing the jailbreak scene are idiots. Installous users are ruining iOS developers because many people are turning there instead of the App Store, losing the developer money. It's all fair to try the game/app in Installous, as long as if you like it, you go buy the game/app from the App Store.
Anyway, on topic, a fix I found was to remove Installous, then remove the Hackulo.us repo. Close Cydia (from multitasking too) then reboot. Go back and re-add the Hackulo.us repo and reinstall Installous.
The reason the API error is occuring is because of JailbreakMe 3.0 - with the jailbreak being so easy to use, many people are trying it then finding out about Installous, in turn overloading the server, giving everyone the API error. Try getting on in off-peak times, or simply wait a few days, or a week or two, and it should be running fine by then.