Changes since SOA version
IXFR is a zone transfer request that returns changes since a specific SOA version. Enter the SOA version to request changes since then.
Not an apex domain
The domain g3-2904-ixf-ar5.router.de.clara.net.router.uk.clara.net is not the root of its zone.
Instead, try zone transfer on router.uk.clara.net .
Instead, try zone transfer on router.uk.clara.net .