Update: it seems that the socket is ok (well almost, when the tablet fell I had to push a tiny amount of foam behind the negative pole with a needle, in order to make the plug fit snugly) since the car charger can charge just fine.
It's probably the plug, a cheap usb ca-100 knock off I used to replace the broken original plug (that worked for some months but it finally stopped working).
My frankenplug worked for a couple of weeks but it couldn't withstand the wear.
Or maybe it's the combination of a half-broken socket with a so and so quality plug.
It's worth noting that the car charger is also from dealextreme and it worked very well for a long time, but then I don't use it as much as the wall charger.