On the Identity Toolkit API Settings page on Google API Console I have provided all the URLs and Rollout %. I see the client and server configuration snippets and the blue Save button. No errors reported. Click Save. It goes to Saving... for a second and returns. Nothing is saved. Go back and revisit - everything is empty. Correspondingly I see the error when trying to execute the toolkit as well with CONFIGURATION_NOT_FOUND error.
I actually had the same issue yesterday evening. Retried this morning ... works again. Looks like it was just a temporary downtime.
Hope it is solved for you also.