It´s maybe the algo that´s the problem. You could test Solo Mining using Scrypt Algo with your GPU - or install Wallet 4.0.3 on another Computer and sync this Wallet using "addnode=ip adress of your 6.14.2" for a real fast sync and test this. One problem could be Segwit as the the old wallet dosn´t support Segwit.
Thanks for the input, hallo_frosch. I was using skein when I tried it, and it seems weird that the server wouldn't work with all algos, but it's worth a try anyways. Gonna test it when I have the time. I had to go back to 4.0.3 to solo mine, no issues with that version.