Traceback (innermost last):
  ...
  Module zope.contentprovider.tales, line 77, in __call__
  Module zope.viewlet.manager, line 112, in update
  Module zope.viewlet.manager, line 118, in _updateViewlets
  Module my.product.viewlets.industry, line 29, in update
  Module Products.ZCatalog.Lazy, line 190, in __getitem__
  Module Products.ZCatalog.Catalog, line 121, in __getitem__
  Module ZODB.Connection, line 860, in setstate
  Module ZODB.Connection, line 901, in _setstate
  Module ZEO.ClientStorage, line 815, in load
  Module ZEO.cache, line 143, in call
  Module ZEO.cache, line 494, in load
  Module ZODB.fsIndex, line 125, in get
TypeError: mybrains.__cmp__(x,y) requires y to be a 'mybrains', not a 'NoneType'

I’m getting to the point, since it had something to do with the deployment. Some packages were deployed as develop eggs on the instance. Only a few files were created with the wrong filesystem permissions, so neither Zope nor ZEO were able to access them. After changing the permissions the errors were gone. So be sure to check all filesystem permissions and if the processes have access to them. If that doesn’t solve it, be sure to also check if not your database is corrupt.