Sometimes when the page loads, at.responsivemenu.js will apply a click event handler to the responsive menu toggles multiple times. If that number is an even number, clicking the toggle will result in the first handler changing the state of the menu, then the second handler changing it back. I believe it has to do with how Drupal now handles Drupal.behaviors apply functionality with bigpipe causing our themes behaviors to be applied twice in some circumstances.
These steps work for me, with the important part being that I'm logged in. Anonymous visiting the site the hamburger menu item only seems to get a single click handler attached, though I've had reports from users that logging out after having logged in does cause the issue to occur sometimes. (caching somehow?!!?!?)
Use drupal/once to ensure we only add the click handler once.
I am unsure how the minified js files are compiled, so my patch comments out at.responsiveMenus.min.js and uncomments the non-minified version. I'll make a merge request with my changes, but if someone could minify the resulting at.responsiveMenus.js we can undo that change in at_core.libraries.yml.
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I am experiencing a similar problem. The responsive menu doesn't open clicking the Hamburger toggle. I am not sure if it is the same cause though.
This is my site: https://www.pflanzini.de/
Can you check if this is related?