backup won't backup a certain virtual server because it has an alias domain name

hello can you please tell me a fix for the automatic backup issue "A new format backup can only be done when all virtual servers have a home directory"

this is because i use an alias for one of the servers...but it should be able to backup the home directory its not an alias meaning .... i have a real home directory on server /home/thisdomain.com/ and then an alias domain to it ..... all i want to do is backup he main domain but whenever i select it i get this error

any advice?