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 u7sdo7b3nf373gjkm3rvar9bceaf6pm5.sk.eserver.net is not the root of its zone.
Instead, try zone transfer on sk.eserver.net .
Instead, try zone transfer on sk.eserver.net .