Hello Eleanora,
thank you very much for your reply.
I use the LT Medical OnePage template (https://ltheme.com/project/lt-medical-free-responsive-hospital-medical-onepage-wordpress-theme/ - but the paid version). This is on Joomla 4.4 and PHP 8.1. And as I wrote, the template is used only for the menu, the rest of my page is all SP Page Builder and Helix.
It seems that there is indeed an effect caused by Helix. Namely the blur effect when the off-canvas menu is displayed.
Also, the behaviour seems to be defined by the Helix template, that when clicking on a menu item in the off-canvas, the page is scrolled to the corresponding position on the page, but the menu remains open.
Do you have any idea how I could simply adjust these two points in the helix template?
I have also placed an enquiry with LTHEME support, because I have other problems/unattractiveness on small screens. But I'm still waiting for an answer.
Kind regards
Ernst