Server Error (5XX) – No longer able to connect.

So I’ve been using the Discord Login for my website for at least a month or two without much problem. A few days ago it stopped working, however. Users are unable to log in now.

Clicking the “Login With Discord” button will direct you to the Discord Login Page, like it should. After entering info and clicking the final Login Button you’re redirected back to the website, just as normal. However, the user never successfully logs in, it seems. The button remains and doesn’t switch to the “Hello NAME! and Logout?”

Clicking again, users are directed to the following Error Page:
ERROR: Please Go back and Refresh the page and try again!
If you still face the same issue please contact your Administrator.

No changes were made to any settings in the plugin or Discord itself. Going through the process again to double check, everything looks perfectly fine. Attempting to clear any cache changes nothing.

Google Search Console gives me the Server Error (5XX) message for the redirect(?)

What could be causing this issue and how do I fix it?

The page I need help with: [log in to see the link]