This is what I have (now):
Traceback (innermost last):
Module ZPublisher.WSGIPublisher, line 181, in transaction_pubevents
Module ZPublisher.WSGIPublisher, line 390, in publish_module
Module ZPublisher.WSGIPublisher, line 285, in publish
Module ZPublisher.mapply, line 85, in mapply
Module ZPublisher.WSGIPublisher, line 68, in call_object
Module plone.z3cform.layout, line 61, in __call__
Module plone.z3cform.layout, line 45, in update
Module plone.z3cform.fieldsets.extensible, line 62, in update
Module plone.z3cform.patch, line 31, in GroupForm_update
Module z3c.form.group, line 145, in update
Module plone.app.z3cform.csrf, line 21, in execute
Module z3c.form.action, line 98, in execute
Module z3c.form.button, line 301, in __call__
Module z3c.form.button, line 159, in __call__
Module plone.app.registry.browser.controlpanel, line 60, in handleSave
Module z3c.form.group, line 116, in applyChanges
Module z3c.form.group, line 82, in applyChanges
Module z3c.form.form, line 51, in applyChanges
Module z3c.form.datamanager, line 91, in set
Module medialog.controlpanel.controlpanel, line 32, in __setattr__
Module plone.registry.recordsproxy, line 48, in __setattr__
Module plone.registry.registry, line 45, in __setitem__
Module plone.registry.record, line 79, in _set_value
Module zope.schema._bootstrapfields, line 295, in validate
Module zope.schema._bootstrapfields, line 510, in _validate
Module zope.schema._bootstrapfields, line 344, in _validate
zope.schema._bootstrapinterfaces.WrongType: (RichTextValue object. (Did you mean <attribute>.raw or <attribute>.output?), <class 'str'>, 'value')