Migrate between Controllers/Cloud Key with "Server Reject" on adoption

Ran into this issue when attempting to migrate a network from a Cloud Key Gen1 to our Unifi Controller (manage.mynet.care). I SSH'd into the router and issued a set-inform command for our inform URL, but immediately got a status of Server Reject when I checked the info command again.

Solved by following this post: https://community.ui.com/questions/Inform-Error-Server-Rejected-Readopt-a-UniFi-Router-Without-Losing-the-Config-4-4-36-5146617/fd8fc2df-0f7e-433f-be25-0e07d31c927a

Specifically:

ssh into the router
mv /config/mgmt /config/mgmt.bkp
reboot
repeat #1
run set-inform http://unifi.yourdomain.com:8080/inform
readopt on the UniFi Controller
Router is readopted and reprovisioned