After hours of debugging I finally found a major bug in subsites module:
When I created a page of the type "Subsites Virtual Page " I wondered why one could only select pages from the 'Main site' (and not form every subsite that existed). In fact there is a dropdown labeled "Subsite" where you can select the subsite you want to choose a page to link to.
I filed a bug ticket where I also posted a fix for the bug:
What IÂ´m asking myself: HasnÂ´t anyone using the Subsites module noticed this bug? I couldnÂ´t find anything about in in the forum...