The key in question is stored at keys.openpgp.org

Can you please try the following?

> FINGER_PRINT=475F3B8E59E6E63AA78067482C7B12F2A511E325
> gpg --keyserver hkps://keys.openpgp.org --recv-keys $FINGER_PRINT

as mentioned in https://github.com/qos-ch/slf4j/blob/master/SECURITY.md