Issue with adding a domain to a secondary DNS

I would like to add a secondary nameserver to my VPS in settings, but I'm unable to do so. Whenever I try to add it, I get an error message saying, that "This domain is already configured on secondary dns server". Is there a way to check what secondary DNS server is this domain configured on? Currently my main and only nameserver is hosted on the VPS, and there are no other NS records for that domain which I've confirmed by using third party tools. There are also no domains on the list in the "Secondary DNS" section of the VPS' settings. Did anybody encounter such problem? Is it my fault/misunderstanding, or is the issue being caused by something else?
I've tried to contact support few days ago but didn't get any reply as of now. Any help would be appreciated.