In adition to what @Taleman already posted, when you use a subdomain of the same domain as name servers, then you need to set glue records at the domain registry and if you use dglue records before, then you must ensure to alter them at the domain registry (not in ISPConfig, glue records are set at the registry and not on your server). Without glue records, such a zone could not be found due to the hen <> egg problem, the zone is required to resolve that record, but without that record, the zone is not found. But this refers only to the special case when a zone uses subdomains of itself as its name servers.
Hello Till, thank you for your answer, I apologize for blaming the migration tool kit, it's solved now. I had already made the changes in the registries that you mention in your comment, only that for some unknown reason these changes were not being applied correctly, until I contacted the registry's support area. Again, sorry, in my defense I could say that I was very confused by not finding the solution to this problem.
