MediaWiki 1.28, Visual Editor 0.1.0 (93528b7).
The script load.php?debug=false&lang=ru&modules=startup&only=scripts&skin=vector&* which is invoked from Special:Upload begins with:
/* [da4dbb21224ac8367486c80b] /w/load.php?debug=false&lang=ru&modules=startup&only=scripts&skin=vector&* BadMethodCallException from line 845 of /var/www/wiki/w/includes/session/SessionManager.php: Sessions are disabled for this entry point */ if (window.console && console.error) {console.error("[da4dbb21224ac8367486c80b] /w/load.php?debug=false\u0026lang=ru\u0026modules=startup\u0026only=scripts\u0026skin=vector\u0026* BadMethodCallException from line 845 of (path to MediaWiki)/includes/session/SessionManager.php: Sessions are disabled for this entry point");}
It writes the message to the console log.
Switching Visual Editor off stops this.