I thought I had done something wrong when configuring pkgdb2, cf: https://admin.stg.fedoraproject.org/pkgdb vs https://admin.stg.fedoraproject.org/pkgdb/ but it seems to affect also pkgdb1 in prod: https://admin.fedoraproject.org/pkgdb vs https://admin.fedoraproject.org/pkgdb/
So we have something incorrect somewhere :)
Should be fixed now.
Was in the varnish config.