By the way. I also have this problem when I try to add a contact to a company: error 500: internal server error.
doesn't matter if I use the quicklink on the right in the company view, or the contacts + sign in the information field.
When I add a normal contact, and add it to the company (so working the other way round) it does work, and from that moment on, I can also add new contacts to the company with the add contact button...