Error or bug when sending

Hi!

I use Contact Form 7 v3.6 and am getting this error (In Swedish) when I try to send:

“Det gick inte att skicka meddelandet. V?nligen f?rs?k senare eller f?rs?k kontakta administrat?ren p? n?got annat s?tt.”

Which means: “Failed to send the message. Please try again later or try to contact the administrator in any other way.”

It’s strange though cause when I try it with my name etc. it works, but when this other guy uses it does not!!

I use WP 3.8.5

Do you know what causes this error?

My form is: https://fotokurser.viktorsundberg.se/fotokurs-anmalan/

Please answer as soon as possible since I rely on these forms.

https://www.ads-software.com/plugins/contact-form-7/