[Zope] External editor "timing out"

[Giampiero Benvenuti]

Hello,

when I edit a page template or a dtml method with ExternalEditor I get  
the following errors:

------
Traceback (innermost last):
   Module ZPublisher.Publish, line 126, in publish
   Module ZPublisher.mapply, line 77, in mapply
   Module ZPublisher.Publish, line 46, in call_object
   Module Products.PageTemplates.ZopePageTemplate, line 351, in PUT
   Module webdav.Resource, line 175, in dav__simpleifhandler
AttributeError: 'NoneType' object has no attribute 'refresh'
------
Traceback (innermost last):
   Module ZPublisher.Publish, line 126, in publish
   Module ZPublisher.mapply, line 77, in mapply
   Module ZPublisher.Publish, line 46, in call_object
   Module OFS.DTMLMethod, line 378, in PUT
   Module webdav.Resource, line 179, in dav__simpleifhandler
PreconditionFailed: Condition failed.
-----

Please notice the error happen when I try to save the documents after  
more or less 10 minutes since the last save. No error happen If I save  
the document after a minute or two since I opened it.

Zope Version (2.13.12, python 2.7.1, linux2)
Python Version 2.7.1 (r271:86832, Sep 13 2011, 19:13:17) [GCC 4.4.4  
20100726 (Red Hat 4.4.4-13)]
ExternalEditor-1.1.0
ZopeEditManager 0.9.7 Mac osx

Can someone help me up with this issue?

Best regards,

Giampiero