Error - Bad Request

We have the following situation,
if we access one registered domains for the webmail in port 20000 for example:

http://www.realestateindr.com:20000

we have this error:

Error - Bad Request
This web server is running in SSL mode. Try the URL https://panel.onemax.com.do:20000/ instead.

if we access
https://www.realestateindr.com:20000
then we have a certificate mismatch error.

is there a way to access a hosted domain for webmail in port 20000 like http://www.realestateindr.com:20000

thanks, regards

Benton