Sending contactform doesn't work anymore

Dear…,

Unfortunately sending the contact-form doesn’t work anymore. When I click the “send-button”, you’ll be redicted to:

http://www.[SITENAME].nlhttp//[SITENAME].nl/contact/#cntctfrm_contact_form

So, something is putting the site in front of the original page.

Any suggestions? It worked great before. I think because updating to another version or updating another plugin something went wrong?!

https://wordpress.org/plugins/contact-form-plugin/