I get the same issue:Ii can perfectly build a ship, but when I hit "release", it is stuck. This happens not only in orbit but also when using an "extraplanetary" launchpad on kerbin. I tried playing around a little bit in order to locate the problem but haven't been successfull. It seems to depend partially on the ship you're trying to launch. For example when I try to launch the stock ion-powered spaceprobe, it gets stuck. But when I remove the radioisotope generators, it doesn't get stuck. But it isn't completely reproducible: I had a ship that got stuck in one savegame but not in another one. BTW: I am using EL with OKS/MKS - so maybe it's because of the alternative launchpads?