@Gblenn 

there is some change on port when upgrading device, Im not sure when it happend bur device itself need access to management port on controller. deafult TCP/8043 on software controller. so chech this port,

if closed upgrade is not posible.

  @MR.S There is a note on changed ports, but it's since v5 (5.x.x). And I don't really see that it's relevant for anything other than when traversing a Firewall/NAT. In my case though, everything is on the same LAN.

As the port change is in reference to 5.x.x and I was running 5.9.9 before, the ports would be the same as after upgrading to 5.12.7, where I started having the problems (including the sluggish behavior). And now I am on 5.9.31 which works perfectly fine, in all regards. And all devices are adopted, that part didn't present any problems regardless of version.

I too am having difficulty checking for updates with the latest controller running in a docker container (with network=host)

In the logs I see this Java exception

  @Hank21 Thanks for replying, but as you can see in my inital post, there are no problems upgrading when on earlier relases of v5 (v5.9.9 and 5.9.31). As soon as I upgraded to 5.12.7 it fails the upgrade... Downloading a file and trying manually from Config menu for the devices, gives the same result so we can rule out firewall issues, which anyway show nothing in the logs at the moment of testing (pfsense). 

Unfortunately I can't provide any screenshots as I have already upgraded my devices after going back to 5.9.31. But the error message is exactly as I stated, and appears in the UI at the top after a short while. The device list shows Connected for all devices, and then the device being upgraded changes to "Upgrading" and then it fails with "Upgrading EAP635 failed because the upgrade file is invalid. Please select a correct file."

With regards to the solution in the link you provided, there is nothing applicable there as I am already on v5... and all devices adopted since way back. 

If there is a way to downgrade for example an AP to an older version I could do some more testing, but I feel I tried enough already that I have proven to myself that 5.12.7 is broken!

And it's anyway obvious that things are not working well with that version as it is really slow (on Chrome and Firefox) compared to previous versions.