I'm actually not quite sure, I would have to look at the package. I had found another related GitHub issue upstream.

https://github.com/linkwarden/linkwarden/issues/244

That is how it at least works for me. But I am using Chrome and not Firefox.

image.png

Edit: just tried out of curiosity on Firefox and there the SSO login seems to be broken and I am seeing the same error message. There is also a discussion on their issue tracker about this.