TypeError: iteration over non-sequence

You will get this page when accessing the site:

Site Error

An error was encountered while publishing this resource.

Error Type: TypeError
Error Value: iteration over non-sequence

You can login to Zope management interface. But viewing sites through it is not possible. You will receive a traceback to the console similar to the following if you have started Zope process on foreground:

2008-11-09 22:53:13 INFO Zope Ready to handle requests
2008-11-09 22:54:50 WARNING OFS.Uninstalled Could not import class 'ATSETemplateTool' from module 'Products.ATSchemaEditorNG.ATSETemplateTool'
2008-11-09 22:54:50 WARNING OFS.Uninstalled Could not import class 'SchemaEditorTool' from module 'Products.ATSchemaEditorNG.SchemaEditorTool'
2008-11-09 22:54:50 WARNING OFS.Uninstalled Could not import class 'SchemaManagerTool' from module 'Products.GenericPloneContent.SchemaManagerTool'
2008-11-09 22:54:50 WARNING OFS.Uninstalled Could not import class 'FormGenTool' from module 'Products.PloneFormGen.tools.formGenTool'
2008-11-09 22:54:50 WARNING OFS.Uninstalled Could not import class 'TemplatedDocument' from module 'collective.easytemplate.content.TemplatedDocument'
2008-11-09 22:54:50 WARNING OFS.Uninstalled Could not import class 'FormFolder' from module 'Products.PloneFormGen.content.form'
2008-11-09 22:54:52 WARNING OFS.Uninstalled Could not import class 'IDropdownSpecific' from module 'webcouturier.dropdownmenu.browser.interfaces'
2008-11-09 22:54:52 ERROR Zope.SiteErrorLog http://localhost:8080/lsm
Traceback (innermost last):
  Module ZPublisher.Publish, line 110, in publish
  Module ZPublisher.BaseRequest, line 424, in traverse
  Module ZPublisher.BeforeTraverse, line 99, in __call__
  Module Products.CMFCore.PortalObject, line 94, in __before_publishing_traverse__
  Module zope.event, line 23, in notify
  Module zope.component.event, line 26, in dispatch
  Module zope.component._api, line 130, in subscribers
  Module zope.component.registry, line 290, in subscribers
  Module zope.interface.adapter, line 535, in subscribers
  Module zope.component.event, line 33, in objectEventNotify
  Module zope.component._api, line 130, in subscribers
  Module zope.component.registry, line 290, in subscribers
  Module zope.interface.adapter, line 535, in subscribers
  Module plone.browserlayer.layer, line 18, in mark_layer
  Module zope.interface.declarations, line 848, in directlyProvides
  Module zope.interface.declarations, line 1371, in _normalizeargs
  Module zope.interface.declarations, line 1370, in _normalizeargs
TypeError: iteration over non-sequence

notice the 'Could not import class' message.