Unfortunately there is a bug in Mendix 18.104.22.168 with GZIP. The index and login.html aren't being zipped from the theme folder with the deployment of the project.
The solution is very simple.
Install 7-Zip: http://www.7-zip.org/
GZIP the html files in your theme package and include the gzip files with your theme package. Keep the html and GZIP files in your theme package together. Re-deploy and it should work again.