stephan5 Hi
You're right, JA Morgan comes with old bootstrap version which may have conflict with the new version in Kunena.
You can try this custom css:
.com_kunena #t3-mainnav {
padding-top: 0;
padding-bottom: 0;
}
.com_kunena #t3-mainnav .container {display: block;}
.com_kunena #t3-mainnav ul.navbar-nav {display: block;}
.navbar-nav .mega .dropdown-menu {position: absolute;}
.com_kunena .dropdown-toggle::after {display: none;}
and see how it goes.