IIt seems like these new themes have a template file,
tiki-show_page.tpl, that is not compatible with
TikiWiki v1.7.5. With this file in the
templates/style/<theme> directory, user menus will not
show the content of the menu, just the title.
Simply removing this particular template file from the
theme(s), and use the "global" template instead
"solves" the problem. I haven't tried to figure out
what is broken, but this solution is good enough for me.