Login bug that we are not able to recreate and fix…

Hi everyone, my company owns an online messaging product that allows end-users to interact through SMS, web chat bots, social bots, etc.

We work with resellers and white label partners to offer our product to their clients under their brand (if they prefer)…Once we come to terms, we set up a reseller dashboard for them which is where our issue lies. Upon signing the reseller up, they receive an email with their login credentials and the link to sign in. The reseller clicks the link, enters the credentials and then the username and password fields disappear. It's not prompting an incorrect username or password, it just disappears without the page refreshing.

When a login loads successfully, there is a spinning loading wheel for 1 second (this doesn't pop up when loading fails). Javascript is allowed on the browsers that are experiencing this issue as well. Any ideas?