[Plugin: Wysija Newsletters] textStatus:parsererror

I′m trying to get the plugin run. But there is a problem with thes subscription form. It only works when I′m loged in as administrator.
Users how want su subscibe get this error:

textStatus:parsererror
errorThrown:SyntaxError: JSON.parse: unexpected character
responseText:

I already disabeld all other plugins put the error won′t disappear?
Does anybody have an idea?

Thank you in advance

Thomas

https://www.ads-software.com/extend/plugins/wysija-newsletters/