I am not using a filler nor browser forms. I opened an xml file with XML Notepad and sure enough the href is pointing to the form template in the same library http://server/site/library/Form/template.xsn where the form is submitted.
I changed the link to point to another library where the content-type is stored when it's published, and now it opens correctly. I was under the impression that the template was also published directly to the library - but based on the behavior, it only is on initial publishing and it overrides the former template (that was not a content-type) .
I used the "Relink Documents" from the View menu to relink the other xml files, now all is well. :) Thank you for the tip!
Doreen