sub-page navigation collapses when one of these subpages is to display the blog

I want a ‘page’ to be the place where i display the Blog (that is, i chose this
page to be the blog page in the backend (Options -> Write).

The page is a sub-page of a root page, and i use jpmcc_fold_page_menus_widget.php
to display the navigation.

Now every time i call this blog-page it displays the chosen posts properly, but
the navigation collapses. I cannot make this currently displayed blog-page
appear in the navigation, it is collapsed right after the page has loaded
only showing me the first level pages, no subpages.

The parent page still carries the classnames ‘current_page_ancestor’ and ‘current_page_parent’,
but its subpages are not reflected any longer in the navigation tree, so the page with
the class ‘current_page_item’, even the whole list of sub-pages, is not in place.

I first thought that this is similar to this post, where Otto42 proposed a solution that
worked well for me – but adding ‘wp_reset_query();’ didn’t change anything in this case.

Does anyone have an idea how to get these subpages displayed or where i might look for an answer? Thank you.