I have pretty much the same issue. I'm stuck as Trevor. I just completed the mission "Derailed", but I cannot switch to Michael or Franklin. I also cannot start the Minute Men mission in Paleto Bay, the Rampage with the soldiers (near the military base), or the Trevor mission in Los Santos (which may be by design). Am I supposed to figure out a way to avoid those gunmen and start the Trevor mission?
I have a feeling there's something wrong with my game. It happened immediately I first played online. Any help would be greatly appreciated. Thanks.