DNS troubleshooting tool incorrectly reporting "ai." as an invalid hostname
Navigate to Diagnostics=>DNS Lookup (found at /diag_dns.php). Enter any TLD that should work as a single-label domain, "ai." for example. Include the terminating dot that indicates you mean an FQDN. This is a valid hostname. You will get a response saying otherwise.
I have tested this on 2.4.4-RELEASE-p1 and -p2. It is not an issue with ubound. Using dig at a command line does show the hostname (with terminating dot) to resolve just fine. The problem is at the diagnostic feature web page.