I have just updated my 2866ax to the latest FW 4.5.0, and the router now reboots itself every 2 1/2-3 minutes.
The firmware has been reinstalled a couple of times, but the issue persisted. I have reinstalled 4.4.6.2 and the router is working again.

Made that same jump myself - router has been up for 9 days 16 hours so far...(got the update from the international site).

Not using mesh wi-fi by any chance ?

I have a AP903 and AP1062C, but both are connected via wire to the 2866ax and have no mesh between them.

OK - was worth the question!

Maybe worth double checking mesh isn't enabled on the 2866 - even if you're not using it.

None of the devices had mesh enabled.
I did try unplugging all the devices from the router, including the AP's and a couple of Draytek switches and it worked fine. As soon as I plugged anyone of them back in it fell over again

I went to update my 2866 last night & got worried when I saw your post as I too have an unmeshed AP903.
The upgrade ultimately failed & I needed to recover using TFTP so is there possible a problem with the image itself?

Out of interest, how long did the upgrade process take for you guys as mine timed out after 15-20 mins, then when I restarted my browser I was eventually able to get back in but it said that it was corrupted. The reboot it suggested then didn't restore the previous firmware, hence the TFTP.