HOT prongs on 30A 240V electric dryer socket (10-30R Socket)
The reason that certain sized wire must be used for a given amperage is because the wire gets hot when electricity is pulled through it.  That said, some equipment from China isn't known for complying to all the regulations we have, and the conductors in the adapters could be undersized.

Many conductors and connectors are rated for 75C, which is 167F, and certainly hot enough to burn you.  However I don't think anything that you can easily touch should get that hot while in use, so I would assume that something is wrong.  I would suspect the adapter first, but you should also measure the amount of current the equipment is using.  It could be pulling a lot more than you think (closer to 7000W) which would overheat an adapter that was probably meant for closer to 4000W.

If you wanted to tinker with the wall outlet, you can make sure the wire clamping screws are tight, and they have a good, solid connection.  The dryer works, and the computers work, so I don't think anything is "wrong" with the wiring itself.