Update: Re-added the Social app and it solved itself.
I just installed NC15 and had no errors. I then installed the new Social app, and got a message that it cannot resolve Webfinger. I have now removed the Social app, and when I do a security check, I get this message:
" Your web server is not properly set up to resolve “/.well-known/webfinger”. Further information can be found in the documentation."
I looked into my .htaccess file and it looks fine, so I am not sure what’s causing this error.