rails 4.2.8 - stylesheet_link_tag throws ActionView::Template::Error: no implicit conversion of nil into String

I'm trying to update a rails app which runs totally fine if I use rails 4.2.4 (the version it's been developed on) upto rails 4.2.7

As soon as I switch to rails 4.2.8 or later (tested all versions upto 4.2.11) I get

Failure/Error: = stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track' => true

ActionView::Template::Error:
  no implicit conversion of nil into String
  # ..../actionview-4.2.11/lib/action_view/helpers/asset_url_helper.rb:140:in `join'
  # ..../actionview-4.2.11/lib/action_view/helpers/asset_url_helper.rb:140:in `asset_path'

Having a loot at the actionview-4.2.11/lib/action_view/helpers/asset_url_helper, I can see that the error happens in

File.join(relative_url_root, source)

where

pry(#<#<Class:0x00000009a9aa98>>)> source
=> "/assets/application-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855.css"

pry(#<#<Class:0x00000009a9aa98>>)> relative_url_root
=> {}

As soon as I remove all stylesheet_link_tags and javascript_include_tags, the error does not show up anymore (but of course all my styles and javascripts are gone).

Here's the relevant part of my app/views/layouts/application.html.haml

!!!
%html
  %head
    %meta{:name => "viewport", :content => "width=device-width, initial-scale=1.0"}
    %meta{:content => "text/html; charset=UTF-8", "http-equiv" => "Content-Type"}
    %title BrotUndButter
    = stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track' => true
    = javascript_include_tag 'application', 'data-turbolinks-track' => true

Any ideas?

I've searched the net for hours, but could not find anything helpful.
and this https://stackoverflow.com/questions/16044008/no-implicit-conversion-of-nil-into-string
does not help at all. This bug should not affect rails 4.X.Y at all. Anyway I tried to set
config.relative_url_root = ""
in my application.rb but that did not make a difference.