Please make sure to have your Python exceptions enabled. 99% of all interface errors, especially when stuff is missing, is because an exception is getting in the way. Just describing the symptoms doesn't help me find the cause.
I don't want releasing civs to become too much of a beneficial strategy. Otherwise it would be too easy to conquer your enemy, release them and gain a friend. Releasing a civ should only be a last resort in case you can't take the expansion penalty, and come with a cost.Last but not least a ssuggestion: when releasing a civ, it should have at least a huge relation bonus to you. I've released morocco because i didn't want to keep that crappy city, and they hate me >.< i regretted releasing them, fack them!!