Singer XL 420

Hi everyone,
I’m really hoping for some help—I’m at my wits’ end.
My Singer XL 420 (which I just bought, but without a warranty) keeps displaying an error message after sewing a buttonhole and won’t move at all.
It beeps, and “04” is always lit up on the control panel.
According to the manual, I’m supposed to reset the buttonhole lever, but it’s already back in its starting position and nothing happens.
Could it be that I’ve overlooked something? I used to sew with an older Veritas machine and am still getting used to this one.
Maybe someone else has had this problem before and can give me a tip—I’d be so grateful.
I can’t imagine I’ll have to take it to a repair shop just yet.
I haven’t found a customer service number to call yet.
Thank you very much in advance.

http://www.singerdeutschland.de/impressum.html 

There’s a phone number in the legal notice. Maybe they can help you.