RWExtras Compact Form (error in php not defined)

hello, just downloaded (paid version) this form stack from @willwood RWExtras and used on my test-site. First the stack won’t work on a stacks page if there are other similar stacks around (doobox Contact Form, stacks4stacks Formulate) but after putting the Compact From stack on a new stacks page it was visible on the published page.
Now while testing the page I get an error notice after clicking the send button:

Notice: Undefined variable: error in /srv/www/vhosts/ on line 39

This message is displayed in the browser window instead of showing a success message on the webpage the form is placed. How can I fix this?


ps the test-site is running here.

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