Hi, I am actually also German, but I grew up in America and just recently moved back to Germany.

I get a 404 when I try to post the form... so I couldn't test it....
As far as I can tell, only reference number, name and email are required fields (or what are the red stars for). Or does the red "attach" mean that pictures are also required?
You'll have to modify the php script. You said you bought it, so I don't know if you have the rights to post it or a small section of it here, but to help you we need to see some code.

Schöne Grüße und viel Glück aus Deutschland!!!