OK, I finally figured out what was wrong with it!
And it was so darn stupid that I'll describe it in as much detail here in case others have the same problem.
On the lens side of the teleconverter, there is a bent piece of metal that has a 90° bend in it that serves as a hook to drive the lens' aperture control lever. On the Minolta mount, it's usually a thick piece of round stock and it's quite sturdy. What happens is that some people don't clock the lens properly when inserting it on the teleconverter and apply force so that the mount goes-in. That's where the problem is. This type of misuse bends the hook a bit too much and it ends-up scraping against the lens holding metal flange.
The trick is simple: take a pair of long nose pliers and just slightly bend-up the tab that's scraping. This releases pressure on the whole mechanism and allows you to fully separate both parts if necessary. It also makes the actuation a lot smoother, just like it should be.
In my case, I took both sides apart to ensure that everything was OK. I then re-assembled it, clocked both halves to make sure that the transmission lever had full motion and re-tightened the set screws to lock everything in place.
It's now working like new, so I'm adding this to my kit of cheap lenses.
Yay!