It sounds like you and your friends were extremely unlucky. Over last 5-6 years I've had several Nikon cameras and Ive only ever had one problem with memory card slots on a
second hand d7000 (problem solved during repair). The thing is with all manufacturers there may be problems at some point, I'm sure Canon had some problems a few years back that sent people the opposite way. There were a few well known pros who were vocal about it and switched...... and later ended up switching back.
There are a few options, 1/ contact Nikon who may or may not be able to solve the issue, 2/ return both the teleconverter and lens, 3/ send the lens and teleconverter away to be checked (didn't you buy them both from a shop with warranty?) 4/ keep the lens and try the kenko TC, if that doesn't work accept that teleconverters may be a problem, 5/ sell up and move to another manufacturer if you really think it's going to help.
I know it's not an ideal situation, even though all older lenses and teleconverters should be backwards compatible due to the f mount. there is always this chance of problems when trying to use a newer generation camera with an older lens and TC. To be honest there is only one person in this thread who can really help you and that's Dragonfly as they have experienced the exact problem before. The rest of us are guessing at what could help. If you haven't spoken to where you purchased them both from I would contact them to see if they can advise you, at very least it logs you are having a problem that may result in returning the items.