The posts look Ok and they match the disconnects you're trying to use them with - maybe a crappy beer out disconnect or there's something going on with the seal on that post?
If you've got other beer out disconnects, I'd try them to see if that particular one is stuffed or ill fitting.
I've never had an issue with a disconnect before and I'm not an expert on how the disconnect mates with the post - but I'd imagine that if the spring in it wasn't strong enough to oppose the spring in the poppet in the post, you'd see low flow rates and a possible congestion point which might trigger foaming in the line?
Hmm, I suspect this may be the case. The disconnect seems to be ok but when I took the keg into a shop to have it looked at, the guy hooked up his hardware and it worked fine. Will try a new disconnect and see how I get on.