If you find a domain that appears to be blocked by NextDNS by error, please first identify which blocklist(s) is/are responsible by going to you logs tab, and clicking on the (i) button next to the name. As NextDNS integrates with many open source blocklists, it is better to go to the blocklist maintainer directly to request the correction.
If it appears to be a NextDNS maintained blocklist, or you can't find its maintainer, you can go to https://github.com/nextdns/metadata, and search in the open issues if someone did not already complain about this particular domain. If not, you can open a new issue and provide as much information as possible about the domain and the blocklist(s) impacted.
Note that if the blocklist is NRD (Newly Registered Domain), this means the domain has been registered less than 30 days ago. It will go out of this blocklist automatically, therefore, it is not necessary to open an issue for it.