There was a regression introduced in z3c.form 3.2.3. It seems to be affecting many people for some time. Here is an example of a issue that was just reported on collective.documentviewer: https://github.com/collective/collective.documentviewer/issues/50
To fix, downgrade z3c.form to 3.2.1. In your buildout, pin the z3c.form version:
z3c.form = 3.2.1
Then, run buildout, restart your instance.
The bug is fixed here: https://github.com/zopefoundation/z3c.form/commit/ab2080a9cd69797167b09eb984787857bcc4d59e
We're just waiting on a new release.
This is affecting Plone 5 latest and Plone 4.3.6.