The directory does not exist
TL;DR
web_root = /web/htdocs4/username/home/, where username is (at least in my case) my ftp login (they use the following convension: for domain example.com the username is examplecom)
Full Story
I’ve just accidently found out what is the absolute path of the web_root.
I installed TCPDF module and enabled both TCPDF and TCPDF Example. The latter created a link TCPDF example in the second (right) sidebar, and when clicked that link, it showed me the following error:
 Fatal error: Class 'TCPDF' not found in /web/htdocs4/username/home/www/modules/tcpdf/src/TCPDFDrupal.php on line 16