I faced the following error as well, however, my integration was with Okta instead.

In our case, the token endpoint was already set to POST. Based on the Loadbalancer logs, we discovered the issue was because we had enabled DPoP (Demonstrate Proof of Possession) with the application on the Okta side.

The AWS ALB does not support DPoP and as a result, we saw the 561. We turned off DPoP and instead turned on PKCE after which everything was working.