Upgraded site from 5.0.8 to 5.1 and now I can't add content

I've upgraded a 5.0.8 site to 5.1 using buildout and now when I login and try to add content from the toolbar, none of the forms are displayed, I just get a blank area on the screen where the for should be.

Any ideas where I should be looking to debug this one?

Did you run the site upgrade on the web interface? A message to run the upgrade should be available at the root of your site structure ( http://localhost:8080/). Are there any error messages on the console or var/log/instance.log?

Do you have some older addons installed? If your addons require formlib and CMFDefault the issue might be

Try solving it like this https://docs.plone.org/develop/addons/upgrade_to_51.html#cmfdefault-removal
I did not test if that works, I still have a meta.zcml in my project with:

<configure
    xmlns="http://namespaces.zope.org/zope"
    xmlns:zcml="http://namespaces.zope.org/zcml">

  <configure zcml:condition="installed Products.CMFDefault">
      <exclude package="Products.CMFDefault.browser" file="configure.zcml" />
      <exclude package="Products.CMFDefault.formlib" file="configure.zcml" />
      <exclude package="Products.CMFDefault.upgrade" file="configure.zcml" />
  </configure>

</configure>

The only addon installed is plone.app.ldap which requires formlib I believe. Adding "zcml += Products.CMFPlone-meta:meta-bbb.zcml" to buildout has fixed this issue.

Thanks for the swift and expert response!

Interesting... would you mind filing an issue at Issues · plone/Products.CMFPlone · GitHub on this? Did you have any existing content in the site?

I've had this issue on two separate sites, and both sites have existing content which was not affected and could be edited. This problem only affects adding new content.

I've filed a new issue for this at: https://github.com/plone/Products.CMFPlone/issues/2338

The issue is already known and the fix is mentioned in https://docs.plone.org/develop/addons/upgrade_to_51.html#cmfdefault-removal. What is probably needed is mentioning the issue and fix in the main upgrade-guide under 'Known issues'.