ZServer probably picks the cPickle file from a different installation. Try adding the following in your start.py file in the ZServer directory.

import sys,os sys.path.insert(0,os.path.join('..','lib','python'))

Ok, my error was to write: import sys,os sys.path.append(os.path.join('..', 'lib', 'python')) instead of using sys.path.insert(0,xxx). Thanks to Anthony who pointed me in the right direction. But Pavlos is kind too...