v13.5.2 breaks external login with Azure AD

[SimonToxic]

Which Umbraco version are you using? (Please write the exact version, example: 10.1.0)

13.5.2

Bug summary

We are using using Azure AD as a external login for the backoffice.
After upgrading from v13.5.1 to v13.5.2 we get the following error when trying to login to backoffice:

Downgrading to v13.5.1 again removes the issue.
But since v13.5.2 is a security patch we really want to be able to upgrade.

Specifics

This might be a bit more detailed stacktrace:

Steps to reproduce

1. Implement external login with Azure AD
2. Upgrade to v13.5.2
3. Try to login

Expected result / actual result

Upgrading to v13.5.2 should not break external login to backoffice.

[github-actions]

Hi there @SimonToxic!

Firstly, a big thank you for raising this issue. Every piece of feedback we receive helps us to make Umbraco better.

We really appreciate your patience while we wait for our team to have a look at this but we wanted to let you know that we see this and share with you the plan for what comes next.

• We'll assess whether this issue relates to something that has already been fixed in a later version of the release that it has been raised for.
• If it's a bug, is it related to a release that we are actively supporting or is it related to a release that's in the end-of-life or security-only phase?
• We'll replicate the issue to ensure that the problem is as described.
• We'll decide whether the behavior is an issue or if the behavior is intended.

We wish we could work with everyone directly and assess your issue immediately but we're in the fortunate position of having lots of contributions to work with and only a few humans who are able to do it. We are making progress though and in the meantime, we will keep you in the loop and let you know when we have any questions.

Thanks, from your friendly Umbraco GitHub bot 🤖 🙂

[SimonToxic]

Just found this issue about the same thing so i close this:
#17383