Updating a 2.3.5 Site to 2.3.16 makes all pages render as text/plain

Today i've tried to update a site because of the latest security
problems.
But this older app breaks as soon i install anything newer than 2.3.5

When switching to 2.3.6 or later in environment.rb (same with the latest
2.3.16) everything gets rendered as text/plain so all i see is the html
source (which is supposed to be UTF-8) as ISO-8859-1.

I'm also not seeing any errors in the logs nor on console.
At most i get some deprecation warnings about rake tasks in
vender/plugin.

What could cause such a weird behavior?
Does anyone know?