It would seem that in the last release, the code for the background service was updated, but the Settings utility wasn’t and is still using the old invalid URL format for TheTVDB.
This means that if you try to override/identify a series by entering a TVDB ID in the Settings app you receive a 404 error.
I spent some time with Charles Proxy and a hex editor and I can see that the Settings utility is sending an invalid URL. Unfortunately, I haven't found a way to correct it as the URL is being constructed within the code. The Settings app is still using the old v1 API methods for TheTVDB. Support for this is rather sketchy now. Even if you correct the URL (something which can be done with Charles Proxy) the Settings app doesn't handle the response correctly.
I don't think this can be fixed without the source code.
It is still possible to override the images and other metadata through manual input.