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 mx-20.eu-west.atmailcloud.com. is not the root of its zone.
Instead, try zone transfer on eu-west.atmailcloud.com .
Instead, try zone transfer on eu-west.atmailcloud.com .