Missing or moved control panels after migrating from 4.3 to 5.1


After migrating a 4.3.18 instance to 5.1.5, the "Date and Time" control panel is missing in the Site Setup page and the following control panels are moved to a "Plone Configuration" category from their original category (as can be seen from a vanilla installation) :

  • Language (from General to Plone configuration) - it has a different icon too
  • Mail (from General to Plone configuration)
  • Discussion (from General to Plone configuration)
  • Image Handling (from Content to Plone configuration)
  • HTML Filtering (from Security to Plone configuration)

I'm mainly disturbed by the absent "Date and Time" control panel. It is worth mentioning that on the Site Setup page just after the migration, there were warnings advertising unset preferences in "Date and Time" and "Mail" control panels - while those preferences were set in the original 4.3 site!

Besides, during the "normal" (and succesful) migration (I mean before the optional step to convert contents to Dexterity), I got the following line repeated regularly :

Step collective.z3cform.datetimewidget has an invalid import handler

Finally, this does not seem to be related to Dexterity migration since this is true even if I don't install Dexterity and keep ArcheTypes content types.

What can I do to fix this?

As a side note, do I have to care about those control panels that have been moved around in the Configuration page? Is this the sign of something going wrong elsewhere?