Error 400 w/o detail for authorize request

I am trying to implement SSO with Okta OpenID Connect. My app would redirect the user to the Okta login page. But for my request

I just get “400 BAD REQUEST Go To Homepage”. That’s all. No error code. Nothing. All the references suggest checking the error’s detail… but there’s no any slightest suggestion.

I tried to find my request in logs by ID with a query string eq "XyE78MgRAN-bGRH7jAErLwAABYc"

but in vain - the query string should be something different… but how should I format it?

I set up a web application like this: