I couldn’t find any URI on Build_Service_Signer to the openSUSE:Tools repo, but wherever it is most likely it does need an update to http://download.opensuse.org/repositories/openSUSE:/Tools/15.4/. For 15.3 is was http://download.opensuse.org/repositories/openSUSE:/Tools/openSUSE_15.3/. Most openSUSE_15.3 repos were changed to simply 15.4.