CMS Made Simple Forums

I have a site where FormBuilder won't submit when pretty URLs are turned on. When I fill out the form and click "Submit" it goes back to the same form, but all fields are cleared. As a backup, I do have it write to a text file, which happens, it just doesn't send mail, or go to the submission page.

Anyone else see this and found a workaround?

CMSMS: 1.11.10
FormBuilder: 0.7.3

Some relevant results with debug option in config.php?
Any output with activated php error messages?
Or hints in server logs?

Is inline switched on or off, try changing it and in the config file is there a page extension set to either html or / try those if blank.