Problem with modal window and AJAX for Wicket 6.14 and higher

In my demo-application I encountered a problem with calling a modal page:
I have taken ModalContent1Page, ModalContent2Page, and ModalPanel1 from
Wicket examples 6.0.x (Matej Knopp) and integrated them into my demo
application.

Up to Wicket version 6.13 everything is working fine with calling modal
window page (=ModalContent1Page). Beginning with Wicket 6.14 (i.e. and
for 6.15) I'm encountering abnormal behaviour after calling
ModalContent1Page, i.e. instead of the content of ModalContent1Page the
content of my web page (with all panels, links etc.) is appearing and I
can call again ModalContent1Page out of appearing modal window and so
on. Only header and close button of the modal window are correct. With
ModalPanel1 there are no problems occuring.

Unfortunately I couldn't reproduce this behaviour with a simple
quickstart I build. I suppose it has something to do with markup
inheritance.

Any hint would be appreciated.

Thanks,

Armin