Some suggestions here:

• Did you name the folder userforms?
  
• Did you do a dev/build?
  

Quoting UserForms@GitHub:

Installation

• Download the module from the link above.
  
• Extract the file (if you are on windows try 7-zip for extracting tar.gz files
  
• Make sure the folder after being extracted is named 'userforms'
  
• Place this directory in your sites root directory. This is the one with sapphire and cms in it.
  
• Run in your browser - /dev/build to rebuild the database.
  
• You should see a new PageType in the CMS 'User Defined Form'. This has a new 'Form' tab which has your form builder.
  

Also, the more info there is on the error, the easier it'll be to track what's wrong.
Go into dev mode and try accessing the page you want to edit. That will give you the complete error message.

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.

Wow. As easy as that. ?flush=all worked for me, too.

Thank you both for your help!