Error message after submitting post

On my member site I and other users get error messages after submitting a post. In dutch the error message is:

Fout: Bericht niet gemaakt. Neem voor hulp contact op met de sitebeheerder.

In English it says: Error: Message not created. Contact the site administrator for assistance.

How can i find out what is causing this error? Some users don’t experience it…
I am using wp super cache and i turned it off but that doesnot help.

EDIT: I totally removed WP super cache and now it seems to work properly, apparently the wp super cache interfered with the plugin functionality.

 

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