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 protonmail2.domainkey.dcjtzknjnrd6ik2pdedsifjfgofbdt6476n7wbbm5dt7ttlelkorq.domains.proton.ch is not the root of its zone.
Instead, try zone transfer on domains.proton.ch .
Instead, try zone transfer on domains.proton.ch .