booki / objavi error : "conversion fail with a BadZipFile error"

I installed booki and objiva2 on my own ubuntu server (in a virtual box), it all seems ok but when exporting a book the process stops with a BadZipFile error. a .zip file with the book in non compressed html format is store in the <objavi2path>/htdocs/booki-books folder and I assume the errors occurs when the next step tries to read what it thinks is a .zip file to convert it to pdf. I get an url pointing to the (non existing) .pdf file.

I might have missed something in the installation phase, may be a library for zipping the file but I am a little lost here. any help/fix is welcome.

objavi.log is the following

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Starting request for /objavi.cgi
effective query is:
{'engine': 'webkit', 'license': 'GPLv2+', 'title': 'Test book', 'destination': 'nowhere', 'max-age': '0', 'pdf_type': 'book', 'book': 'test-book', 'mode': 'book', 'server': 'en.flossmanuals.net', 'booksize': 'COMICBOOK'}
templates/nowhere.txt
text/plain; charset=utf-8
nowhere
sync
in get_watchers. method 'sync', callback None, destination 'nowhere'
watchers are set([<bound method Context.log_notifier of <__main__.Context object at 0x1766f10>>])
*** Starting new book testbook-en-2011.12.02-19.25.37.pdf ***
notify_watcher called with 'start'
******* got message "start"
fetching zip from http://192.168.1.6/booki-twiki-gateway.cgi?server=en.flossmanuals.net&book=test-book&mode=zip
notify_watcher called with 'fetch_zip'
******* got message "fetch_zip"
Traceback (most recent call last):
  File "/home/colivier/Sources/objavi2/htdocs/objavi.cgi", line 444, in <module>
    main()
  File "/home/colivier/Sources/objavi2/htdocs/objavi.cgi", line 437, in main
    output_function(args)
  File "/home/colivier/Sources/objavi2/htdocs/objavi.cgi", line 332, in mode_book
    page_number_style=args.get('page-numbers'),
  File "/home/colivier/Sources/objavi2/objavi/fmbook.py", line 191, in __init__
    self.store = zipfile.ZipFile(f, 'r')
  File "/usr/lib/python2.7/zipfile.py", line 712, in __init__
    self._GetContents()
  File "/usr/lib/python2.7/zipfile.py", line 746, in _GetContents
    self._RealGetContents()
  File "/usr/lib/python2.7/zipfile.py", line 761, in _RealGetContents
    raise BadZipfile, "File is not a zip file"
zipfile.BadZipfile: File is not a zip file

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Thanks