Use an own name server for domain and sub domains

Hi,

I want to run an own nameserver for our organisation. So I set up BIND with all necessary information and want to change the DNS settings at OVH. So I followed the instructions at https://docs.ovh.com/gb/en/domains/glue_registry/

1. set a glue record (`ns.hackspace-jena.de --> ["37.221.197.87"]`)
2. set two A records which point to ns and ns2 (`ns IN A 37.221.197.87`; `ns2 IN A 109.230.195.70`)

Both steps seemed to work. The last step is to change the DNS server. But when I try to change the servers from the OVH ones to `ns.hackspace-jena.de` and `ns2.jackspace-jena.de` those changes are reset. The operations log just says "cancelling DNS update" without any further information.

Can someone help me and tell what I'm doing wrong here?