The problem is with wrong settings on your server. Check this link (without www.) – http://herzensfreundinnen.de/ and try to subscribe. Subscription works fine.
You need to set the same site address in the settings – go to Settings -> General -> set the same site url for WordPress address and Site address – http://prntscr.com/au38lr
If you still will have problems with it – please contact your hosting provider, they should help you to configure the site.
I?m having the same problem! My site is http://www.natalialopopolo.com
I?ve already set the same site url for WordPress address and Site address
First I had the “loop” and now the email error massage appears.