TYPO3 Core

Example:

config.compressCSS = 1

font-awesome = //cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.css
font-awesome.external = 1

TYPO3 unsuccessfully tries to load the external file from the local webroot and results in "Oops, an error occurred!"

Entry in TYPO3 log:

file_get_contents(<webroot>///cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.css): failed to open stream: No such file or directory

If compression is disabled globally (config.compressCSS = 0) or for the external resource (font-awesome.disableCompression = 1) everything is OK.