I had this issue today too. New 505 on 8.6.0.0 to start. AP started out in this upgrade-only mode, as the first of an instant cluster. Others did the same thing, but once I followed the upgrade instructions above to 8.6.0.2, it gave me the instant interface I expected. I was able to put the second URL into the WebUI upgrade URL window, and that worked. So, to me, these 505's are not behaving properly as shipped... right? Should Instant UI after not finding a controller. Can't find any info in release notes or guides referencing this behavior.
CLI said this, on SSH login:
"Warning: CLI is currently running in upgrade-only mode. Only upgrade operation is supported, other operations may not funcion." (yep it is spelled that way)