I have my zypper repos set to the GWDG mirrors, however I frequently get the error “Valid metadata not found at specified URL” for various repos (currently this is happening for at least games, games:tools, hardware, security and tumbleweed update). I reported it the the operators there, and according to them SUSE are pushing inconsistent repos to the mirror:
unfortunately we cannot do much about it. SUSE pushes us new files via rsync directly and sometimes these pushes are not consistent with the metadata. I have re-synced opensuse/repositories now fully again from SUSE-servers. I hope this helps with the current problem.
Best regards