Domain name extensions .com, .net, and .edu have a special restriction when using custom nameservers (see Change nameservers for my domains for information on registering custom nameservers.) This is a restriction by their registry, VeriSign®.
The restriction prevents circular nameserver references from resolving properly to the hosted website.
What is a circular nameserver reference? — If you create custom nameservers for two domain names, a .com and a .biz, for example, and the domain names use each other's nameservers, the reference is circular.
Example of a circular nameserver reference:
example.com uses: ns1.example.biz and ns2.example.biz
example.biz uses: ns1.example.com and ns2.example.com
Here's the fix: Change the nameservers for one of the domain names.