Inline and defer CSS crashes website (timeout)

Hi,
I’m currently optimizing page speed with the plugin. I’m currently trying to identify all necessary css styles for above the fold which I want to do step by step.

So initially I activated the according option for inline and defer css and simply added the following css code to the box for above the fold css (in order to see a blank page and take it from there):

body {
    background: black;
}

The page initially loads blank with only the html and without CSS (as expected). However from there the page keeps loading without any visual changes until after some time Chrome alerts me that the page is broken. There must be some conflict or perhaps javascript errors which I’m unable to identify at the moment.

Did you hear about this issue before?

Thanks