Domain limits

Hi everyone. My VirtualMin can run about 15 domains with no problem. but when I add 16th domain it display following message at browser:

 
Forbidden
You don't have permission to access / on this server.

Interesting part is if I delete one of my older domain Then last domain magically start to work. ( I have backup to restore ) so issue is not from permission or not having index.php file. They way it act is like there is total limit of 15 domain for VirtualMin but I know VirtualMin don't have domains limit. So some thing seems wrong. some thing seems prevent having more than 15 domains.

All update by Fri Sep 11 13:48:22 2015 are installed. I have CentOS Linux 6.7 with Webmin version 1.760 and Virtualmin version 4.18.gpl my Kernel and CPU Linux 2.6.32-042stab102.9 on x86_64

Any idea how to fix the issue?

Update: I checked Apache error log it shows permission denied.

[Fri Sep 11 13:13:11 2015] [error] [client  the ip] (13)Permission denied: access to /index.php denied

then I tried

 chown -R username:username /home/username/public_html

No luck so far. As I mentioned it look like permission issue but issue is tied to total domains number. because domain start to work as soon as I remove older domain, and if I restore old domain again. then it wont work unless I delete another older domain or my total domains number is less than 16. very confusing. Maybe VirtualMin have a domain limit and I am not aware of it :)

Thanks

SkyMan