Waiting for HTTP-01 challenge propagation: wrong status code '500', expected '200'

[Jaapel]

Hi all. I am running cert manager on kubernetes using a ClusterIssuer. My setup used to work fine until recently.
When I create an ingress , the Challenge remains pending, with an event that reads:
Waiting for HTTP-01 challenge propagation: wrong status code '500', expected '200.

I am not sure how to debug this. Can anyone point me in a direction?
The ingress is using nginx-ingress and my endpoint is behind oauth2-proxy, but that did not matter before.