First of all: You might want to check the logs: both sites logs + server logs + db logs. There got to be some error code somewhere.
Here's a list of things you can try:
Use different admin users for both sites
Check the redirect rules in your htaccess. Especially the www redirect, maybe there is something wrong in your declaration that can confuse the redirection
Make sure the 2 sites have clearly 2 differents domain declaration in your vhost, and that both declarations are pointing to the right folder
Make sure your directory declaration for each site contains the full path (
<Directory /var/www/html/site1>), here is an example:<Directory /var/www/html/site1> Options Indexes FollowSymLinks MultiViews AllowOverride All Order allow,deny allow from all </Directory>Make also sure you have nicely declared your bind zone (zone declaration in named.conf.local and creation of your db.domain.com). If one of them is a subdomain, hopefully you should have made a subdomain declaration in the main domain.
And check again your settings, especially the base_url and trusted host patterns, that can help too.
As you can see there might be quite a few sources to your issue. You may want to make more comparing tests of use cases what happens to one site if I trigger this in the second and vice versa. Hope this can help.