for the testing purpose:
if you use ui:include instead of a:include, does the message is the same?
Yes the problem is the same with <ui:include>.
Whats interesting is, that in the included file there is no such line
as <!DOCTYPE ...>, but it may be automatically generated by facelets
Any other hints?
So, i solved the problem!
I made an mistake which I now realized:
I had a main page template that defined tags like html and should therefore
be the outermost code defined.
The problem was that I had two pages that used that layout and one of these pages was included into the other one, so this is where the
<!DOCTYPE ...> came from which was mentioned on the error page.
Thanks for your reply, anyway!