Thank you for replying. I've installed it as stated. And it's not the first module I've installed. And I tried it now 3 times.
Unfortunately I don't get an error message with dev mode since I can't exit the page.
When I add a new page and I choose the page type "User Defined Form" I click "create" and then I get the error message. Warning at line 670 of /home/httpd/vhosts/xyz.ch/httpdocs/silverstripe/framework/view/SSViewer.php
That's it. I can see the page in my backend, but can't edit it. And I can't access the site of the frontend.
Would you have any further suggestions. Thank you!
Hmm. I think when you click "Create", even without the page loading up, the URL should change.
What I do in these situations when all I'm given is that tiny popup with "Error on line X" is click on the URL in the address bar and hit Enter. That forces the page to reload. Should be given the error message then.