We have seen this with 3.5.2 and older firmwares, you need to power cycle the radio not reboot. Dont ask why, but it does fix the issue to allow an upgrade to take place.
I observe that I had to go physically to the location then
a.) Power cycle the radio unit;
b.) Defaulted the radio unit to factory default settings
c.) after which I was able to upgrade the radio unit from 3.5.2 version now to 4.6.01 version. d.) Then after which I had to reconfigure the radio unit for the link to be back up and running.
I see you got this figured out but for future reference or anyone else in the future with a similar issue.I’ve run into this same issue before and for me it was a browser issue. When I switched browsers I was able to upgrade the firmware. I normally go back and forth between Chrome and Firefox so when I had this issue one worked and one didn’t.