Hi again, I’ve spoken with the Dev team, they have worked with FastCGI briefly in the past, they have since used mod php. They doubt what you’re seeing would be theme related, but they have said that Fastcgi is memory intensive and that it has some limits for the htaccess file, either one could be causing the issue you’re seeing.
They suggest checking in with the host about this since they know what’s best for their servers and have probably seen issues like this before if working with WordPress as a platform. You can also try upping the php memory further and checking if they place any conditions or alterations in the htaccess.
Let us know what they say as this could well help someone in the future experiencing a similar issue. If there’s anything in the error logs which point at anything particular we can answer on please let us know and I’ll relay to the Dev team.