Soundiiz is using 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 are not able to 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, disabled it.
After that, you should also check that your Plex server is reachable outside of 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/
Be aware that we are not able to reach Plex server using Firewall / Proxy / Double-Nat etc. You should first disabled these elements before trying to connect Plex to Soundiiz.
Finally double check that you are using an "Admin" account Plex here to sign in with (guest account could not work).
As long you are passing successfully the login Plex process on Soundiiz but then being disconnected, that means we are not able to reach your server OR your server is taking too long time to respond.