I'm an Linux user, currently running Unbuntu Maverick.   Just updated from 7.18 to 7.36, but when I run the wizard I getting the following error:
ERROR: ERROR: Unhandled exception: <b>TypeError</b>:expected string without null bytes
Traceback (most recent call last):
  File "/usr/lib/calibre/calibre/gui2/wizard/__init__.py", line 773, in accept
    page.commit()
  File "/usr/lib/calibre/calibre/gui2/wizard/__init__.py", line 459, in commit
    dev.commit()
  File "/usr/lib/calibre/calibre/gui2/wizard/__init__.py", line 58, in commit
    cls.set_output_profile()
  File "/usr/lib/calibre/calibre/gui2/wizard/__init__.py", line 47, in set_output_profile
    recs = load_defaults('page_setup')
  File "/usr/lib/calibre/calibre/ebooks/conversion/config.py", line 40, in load_defaults
    r.from_string(raw)
  File "/usr/lib/calibre/calibre/ebooks/conversion/config.py", line 80, in from_string
    d = eval(raw)
TypeError: expected string without null bytes
also, got this when trying to convert to another format:
ERROR: ERROR: Unhandled exception: <b>TypeError</b>:expected string without null bytes
Traceback (most recent call last):
  File "/usr/lib/calibre/calibre/gui2/actions/convert.py", line 109, in convert_ebook
    self.gui.library_view.model().db, book_ids, out_format=prefs['output_format'])
  File "/usr/lib/calibre/calibre/gui2/tools.py", line 40, in convert_single_ebook
    d = SingleConfig(parent, db, book_id, None, out_format)
  File "/usr/lib/calibre/calibre/gui2/convert/single.py", line 124, in __init__
    self.setup_pipeline()
  File "/usr/lib/calibre/calibre/gui2/convert/single.py", line 173, in setup_pipeline
    ps = widget_factory(PageSetupWidget)
  File "/usr/lib/calibre/calibre/gui2/convert/single.py", line 167, in widget_factory
    self.plumber.get_option_help, self.db, self.book_id)
  File "/usr/lib/calibre/calibre/gui2/convert/page_setup.py", line 57, in __init__
    self.initialize_options(get_option, get_help, db, book_id)
  File "/usr/lib/calibre/calibre/gui2/convert/__init__.py", line 70, in initialize_options
    defaults = load_defaults(self._name)
  File "/usr/lib/calibre/calibre/ebooks/conversion/config.py", line 40, in load_defaults
    r.from_string(raw)
  File "/usr/lib/calibre/calibre/ebooks/conversion/config.py", line 80, in from_string
    d = eval(raw)
TypeError: expected string without null bytes
Tried reverting to 7.18, but the same problem happens.   In truth, never really used 7.18 since switching to Maverick, so not sure when the problem began.   No improvement after updating python.