Soundiiz uses the Plex.tv login process to handle the connection to your server.
Be sure to use these Plex.tv Connect credentials when you are trying to connect Plex to Soundiiz. Use this link to create a Plex.tv account / try your login and password=> https://www.plex.tv/
If you cannot load the login form, check if you don't have a browser plugin that could be the culprit, like uBlock Origin (Firefox/Chrome plugin) was the culprit; and if yes, disable it.
After that, you should also check that your Plex server is reachable outside your local network. You have to enable "Remote Access" in your Plex server settings. Here is a tutorial for that too: https://support.plex.tv/articles/200289506-remote-access/
Just to let you know, we cannot reach Plex server using Firewall / Proxy / Double-Nat etc. You should consider disabling these elements before connecting Plex to Soundiiz.
Finally, double-check that you are using an "Admin" account Plex here to sign in with (the guest account could not work).
As long you are passing the login Plex process successfully on Soundiiz but then being disconnected, that means we cannot reach your server, OR your server is taking too long to respond.
Check your server logs to get more information about the issue.