Ok so i figured it out. So the actual error message which was masked / muted by your custom error code in PLL_Admin_Model was that the taxonomy term already existed in the database.
My best guess is that the previews fatal error corrupted the database while adding the data but not finishing up with what ever it is, polylang needed to work.
I deleted the corrupted data manually from the database and now without the rocket plugin which lead to the error in the first place and without the corrupted data i was finally able to add a language ..