Yesterday, on my UniFI USG configuration, I made a small change to a site-to-site IPSec VPN. It broke—I could no longer connect to the remote site. I reverted the change. It was still broken.
At one point I noticed that the site name had changed. Is the controller unable to even keep the site name?
Finally I compared the Pre-Shared Keys between the two sites. Completely different!
LastPass Did It
It turns out that my password manager, LastPass, was filling in the name and password from another site. Once I logged out of LastPass and fixed the Pre-Shared Key, everything worked again.
Watch out for password managers when using web-based management portals!