the socket or the adapter, one of them must be broken...

Same problem here. I've had two 211s that have died within a month of each other. TP-Link lived up to their warranty and replaced them with Nanos that are now up and working. I'm still using one PA-211, but based on the performance of the 211 starter kit, expect it to die sometime soon.

The problem seems to be a capacitor on the circuit board that goes bad. I've seen a couple posts about it on YouTube, and if you're determined, it can be repaired. The part is available on Ebay, or I suppose, a local electronics store. Since TP-Link lived up to their warranty, I didn't attempt a repair. One of my dead ones got returned to TP-Link to verify the problem, the other ended up in a local landfill.

Same here, four out of four have failed and been replaced with Nanos. When the third one failed last month I suggested that they also replaced the surviving one to save postage. They refused so this month I have paid out again to return the fourth failure.