Domain Mapping Plugin not showing mapped domains in his list.

Hi Heroes

We have started a new Multiwebsite but the Domain Mapping Plugin does not show the mapped Domains in his list.

Any ideas welcome!

Setting
Site (1) - sub.maindomain.tld
Site (2) - Main.domain - defined in wp-config as site 1 and blog 2
Site (3) - other.domain

Site is using subfolder structure
Domain Mapping Plugin is installed as well as Prosites

Ideas and Solutions welcome - Acces granted

Kind regards
Andi