I have four ATV3, one configured for plexconnect and tried to update to the new OS. Three have worked, only that one with the plexconnect profile doesn't work.
I've deleted the profile and restarted ATV3, but still no update possible.
Does plexconnect have any influence on the update functionality? Any suggestions?
Yes, PlexConnect blocks the Update service so you only update consciously.
Set you DNS settings back to "Automatic" and then invoke the update. Once the process is finished, just switch your ATV settings back to the IP of your PlexConnect machine.