Acrobat »not responding« - AFP file share

Everything in the chain is involved: server disks, server o/s, server drivers, server network card, hubs, cable, switches, routers, client network card, client driver, client o/s, client app. They all can make choices and trigger problems elsewhere. Sometimes even the protocol design is faulty, but that can be worked around at various levels. So the question is where the most likely focus will get results. It isn't likely to be Acrobat that needs changes. It is best not to think in terms of "XXX has a fault, XXX needs to be fixed, it narrows your investigation to try to decide this too early". It's certainly worth asking QNAP if they have known interoperability issues. Just because it isn't a computer doesn't mean it isn't packed with software that can have bugs or needs updates.

If it were me, I would be running packet capture on the clients, trying to capture the packets for a good exchange and a bad one. But this is limited as the communication itself will be encrypted. Still, someone who could analyse the packets might see something.

Acrobat may be doing something unique, unusual, or even wrong, but at the end of the day if afpfs is hung/not responding, something is seriously wrong with it or beyond. Reading a file isn't just about reading bytes off the network, there are all sorts of different protocols and requests. Could also turn out to be the server - check whether it's just been patched.

Well, the message includes "(afpfs) has been unresponsive for 18s blocking". This is really solid information. Acrobat has asked for something to happen, and there has been no response. Now, Acrobat might be asking for something weird, but no matter how weird, the network disk shouldn't block.

So, it might be a network fault (drivers or hardware) or a Mac OS fault (in afpfs or elsewhere).

I suggest taking it up with Apple. I don't know if you have tried other high-load read/write apps on the same network. They might not show the same issues, but if you can persuade any other app to hang, you would have something very solid to talk to Apple about.