I did - no change. 

I also turned on maximum error reporting but nothing shows
 

Der Link zu dieser Seite funktioniert nicht.
Fehler 403.

Screenshots von Ihren OffCanvas Einstellungen bitte.
Alle Module in der Offcanvas Position mal deaktivieren und nur ein eigenes Modul mit einem Text dort einfügen und erneut testen.

From where are you accessing the site? I may have to turn off the firewall.
And, are you using a VPN? I often get misplaced in terms of location.

If need be, please send me your IP in private message. 

The module in the offcanvas position is a custom module with text, an image and a couple of links. Nothing fancy. 
However, I did unpublish that and put a one-liner 'this is a test' in its place with the same results. 

Also, I elected to not show the offcanvas position at all but that did not fix the mobile menu issue.

Settings of offcanvas in screenshot. Thank you!