Thanks for the detailed info! I will try this when I get a chance, but I don't think it's my issue (unless I'm misunderstanding what those commands do.) While the problem print servers ARE using TKIP, the iPads on the same wlan are not... which is why I have "WPA-Mixed" as the encryption mode.
Troubleshooting further, I realized yesterday there was a fimrware upgrade for these print servers that enabled AES, so I upgraded one with high hopes... with the same result. Said another way, even when using AES, these problem devices still do not work, so I don't think TKIP is the culprit at all, now.
It seems to be something more fundamental that changed between 9.4 and 9.8 (and since I upgraded through the necessary in-between versions rapidly, I have no idea if, say, 9.6 was when they would have stopped working, or if it really is 9.8 that's the culprit.).
The craziest part of all of this is that the devices DO magically start working (after 12am... no idea if that's significant) and work fine until they are rebooted via a power interruption. I suspect these are just terrible wifi devices, yet I cannot escape the fact that they worked perfectly against this ZD at 9.4 and now do not a 9.8, with absolutely zero changes to the clients. We have a mystery, for sure.