Hello again
Now I've solved it! It had all to do with settings in Virtualmin and folder rights and I could see that thanks to the debug mode you suggested.
I was suggested by staff from Virtualmin to create an Alias insted of a domain, and to start with it looked fine, but then I relized I could not install a certificate; so I then created a domain in Virtualmin seconddomain.com, and tried to give it the folder paths and User and Group of the main domain maindomain.com , but Virtualmin interface objected.
I so looked up the seconddomain.com domain config file in
/etc/apache2/sites-available
and made all changes manually, sidestepping the Virtualmin interface.
The first thing I did was to change the SuexecUserGroup to the values of maindomain.com, in my case "#1004" "#1004" to give the correct folder rights
Then I changed all directory paths (except for cgi-bin) from
/home/maindomain/public_html
to
/home/secconddomain/public_html
Remember doing it for both :80 and :443
But when trying to run seconddomain.com there were errors saying (remember I'm still in debug mode)
The directory "/home/maindomain/public_html/var/cache/dev/doctrine" is not writable.
I changed folder rights for /doctrine from 0755 to 0775 and now it is all working.
I hope someone will have use for this information.