For people using Docker, those warnings get saved to userdata/logs/upgrade.log. I noticed this new little feature when I upgraded to the latest snapshots awhile back. That is where you can look for the warnings that users who install using apt get to see when they upgrade.
Prior to recently, there was no way to get those warning when upgrading a Docker container and it is not widely well known that upgrade.log file exists.
The REST API Docs moved in 2.5 M1, I don’t think the Dashboard moved.
I’m not seeing that error either. Do you have the Dashboard running? When you go to your main page http://<server>:8080 does it fail or do you get the list of big old icons showing all the installed UIs. I can’t figure out why it would be wanting to uninstall it in the first place and the error indicates it’s already uninstalled. Curious indeed.
Have you tried Clear the Cache? I know you moved to a fresh container but sometimes even during a relatively clean upgrade stuff can get messed up in the cache in particular. I did run into some errors (not that one) when I upgraded from 2.5 M1 to the Snapshots a month or so ago and clearing the cache sorted it out.