Dynamics Portal - Sign in failed ("Account/Login/ExternalAuthenticationFailed")

Hello,

I'm having an issue when a user tries to sign into the Portal he/she ends up on a page saying "Sign in failed".
The URL of the error page is: "https://D365PortalUrl.... .com/Account/Login/ExternalAuthenticationFailed".

We're using authentication trough Azure Active Directory B2C. It looks like the portal is caching something.

It only happens the first time I open my browser, if I then click Sign In I end up correctly on the B2C url:
"https://D365PortalUrlb2c.b2clogin.com/........."

Has anyone experienced this problem and have any suggestions on how to solve the issue?

Thanks for all you help!
/Marcus Stenswed