Based on the screen shot you shared
Which shows you have the Content-Type set to the same value the error is stating it only accepts, and the fact that this type of issue has persisted in the API connector for sometime (issue being everything is set properly but Bubble sends wrong value and get error) and the fact that Bubble recently made an adjustment (usually based on historical track record means lots and lots of Bugs) I would assume this is a Bug in Bubble.
I think you can close the browser, clear the cache and history, then try again. If that doesn’t work, reach out to Bubble support who will at first say to look at the forum as they don’t help with API stuff, but clearly articulate the fact that you have added the correct Content-Type.