hello kaPHC, marahil isa yan sa buggy implementation ng VPN connection ng android version mo.. si Postern kasi as of version 3.x ay hanggang Jellybean nalng ang supported nya if ang android version mo ay 4.1 below.that's the reason why you can't run it any more.
Suggestion1 : upgrade android (if capable pa)
Suggestion2: downgrade postern (prior to 3.x)