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 nsm_vpn_extern.bvi103.n540-buch-buchenweg-101-gk.ipv6.wtnet.de is not the root of its zone.
Instead, try zone transfer on ipv6.wtnet.de .
Instead, try zone transfer on ipv6.wtnet.de .