Also for note the biggest difference between my evolution light template and all of the default boonex templates is the menu. The menu of the evolution light template is more complex and has more javascript and processing that occurs especially when the browser is resized. The additional memory requirements of the template or the extra script calculations to determine what goes in the more menu or not may also contribute to it in some way.

I fixed the bad gateway error by increasing the fast cgi parameters in the server block for the website. fastcgi_buffers 128 128k; fastcgi_buffer_size 128k; fastcgi_connect_timeout 3000; fastcgi_send_timeout 3000; fastcgi_read_timeout 3000; from the old setting of fastcgi_buffer_size 4K; fastcgi_buffers 64 4k;  increasing the timeout from the default settings may have been the real cure.  Here is the funny thing; the beta site, 1.2.0 beta works fi

NOTE:I have repacked the full install zip file for version 1.2.0. The one originally provided contained a extra folder named Cheetah-master which contains a copy of all of the files from the GitHub repo. If your new install of 1.2.0 has this folder, you should delete that folder. It should not be there.The new copy of the zip file available for download does not contain that unneeded folder.

  Question for Deano; were there any changes in the evolution_light template between the beta release and the full release? None.

Restored the site, applied the upgrade through ftp, and still get the gateway error on switching to the evolution_light template but not on switching to any other template.  Normally this means that Nginx was unable to communicate with php-fpm; often a result of misconfigs in the pool.  The same pool handles the rest of the website.  I could play around with the settings in the pool but why would the evolution_light template required changes and not the others?  Why does it work OK on the beta 1.2.0 site

I am going to restore the VS and then reapply the upgrade and probably use ftp to transfer the files; could have been the file manager in virtualmin.  I did a tarball yesterday using the file manager and it would not complete the archive.  I used the command line to create the tarball and it worked perfectly.

Works fine on my Android phone running FireFox.  WillMonte is seeing the same issue on his laptop running Windows 7 with current FireFox browser.  I am not sure how to track this down. Update, Will stated that it does not work on his mobile phone.

Just checked my 2 test sites with firefox in linux mint 18.3 and both sites load the template fine.https://installtest.cheetahwsb.com This one is a fresh install of cheetah 1.2.0 And https://upgradetest.cheetahwsb.com This one is a upgrade to cheetah 1.2.0 from 1.1.0  

Don't see why. I run the latest browsers as well.

  Have you tried uploading the upgrade files again? My procedure was to download the upgrade zip from your sever to my server and then extracted it on the server and then copied and replaced/merge the files.  Tomorrow I will do fresh install and see if that is any different.