I have monitored what happens here in firebug. Attempting to add any of the following field types (and only these field types) to a form in the CMS:
2. Radio Button
Result in only a <br /> being inserted onto the form instead of the actual form element.
Video of the problem here:
To reiterate - I was having the same problem as the title of this thread after upgrading from 2.2.3 to 2.3.1 + userforms 0.1.0.
The steps I took to this point were:
1. Delete sapphire, cms and jsparty from webroot.
2. Copy replacement folders for these three, plus userforms direcory to webroot
3. Run /dev/build/?flush=1 on the site
4. Contact form has the error as specified in the subject of this thread
5. Go to CMS, delete any Dropdowns from the orignal form
6. Attempt to add new Dropdown to the page using the CMS.
Any further help to fix this? I have tested a fresh install of 2.3.1 with userforms 0.1.0 and all seems fine. The only other real different is the database.