Ok, I figured it out.
I disabled //$contentDirAbs = $_SERVER['DOCUMENT_ROOT'] . '/' . $_GET['wp-content'] . '/webp-express';
instead, I hard coded the path – $contentDirAbs = $_SERVER['DOCUMENT_ROOT'] . '/wp-content/webp-express';
It throws the errs because /var/www/junakslavicin.com is a symlink, not a directory. Therefore, “../../” (returning the 2 directories above) does not work.
Please, make a update of the code, because this is a critical bug.