failed to instantiate file default.master not found.

This was a tough one. Trying to setup a site-definition and then received this error always, when trying to create a site.
My basic site-definition was extended with a module to initialize the masterpage, because I read on a website to do so.
It seems that the error is somewhere completely different. Got to the onet.xml and change the configuration-tag:

Usually most websites do show the onet.xml configuration tag as a relative path. But this change fixed my project.