[Health] No module named transaction

[Laboratorio Verónica Gómez]

Hi everyone,
I have installed tryton 1.6.1 and gnuhealth1.3.2 in ubuntu 11.04.
First, trytond was not where  gnuhealth was looking for, but I managed this with the 'with-tryton' option. 
Now when I try to start trytond it shows the following:

Traceback (most recent call last):
  File "/usr/bin/trytond", line 30, in <module>
    trytond.server.TrytonServer().run()
  File "/usr/lib/pymodules/python2.7/trytond/server.py", line 99, in run
    Pool.start()
  File "/usr/lib/pymodules/python2.7/trytond/pool.py", line 75, in start
    register_classes(reload_p=reload_p)
  File "/usr/lib/pymodules/python2.7/trytond/modules/__init__.py", line 376, in register_classes
    pathname, description)
  File "/usr/lib/pymodules/python2.7/trytond/modules/medical/__init__.py", line 2, in <module>
    from medical import *
  File "/usr/lib/pymodules/python2.7/trytond/modules/medical/medical.py", line 27, in <module>
    from trytond.transaction import Transaction
ImportError: No module named transaction

Any ideas?
Thanks in advance.

--
Verónica I Gómez