I'm having the same issue as @tmchow, with the same error message in the browser console. It is also being reported here
https://www.reddit.com/r/unRAID/comments/khl3s0/issue_adding_peer_to_wireguard/
I've tried several different browsers, and the "apply" button has no effect. Is anyone else experiencing this? This is with unraid 6.8.3 and plugin version 2020.07.10b.