ImportError: No module named psycopg2

I have koo 5.0.3 running on Windows XP, openerp-server 5.0.12 on a debian lenny, serving the client through Pyro.
When I want to create a new project based on a template I get the following Traceback (client message):
Traceback (most recent call last):
  File "Koo\Rpc\Rpc.pyc", line 151, in call
  File "Koo\Rpc\Rpc.pyc", line 140, in singleCall
  File "Pyro\core.pyc", line 392, in __call__
  File "Pyro\core.pyc", line 462, in _invokePYRO
  File "Pyro\protocol.pyc", line 429, in remoteInvocation
  File "Pyro\protocol.pyc", line 454, in _remoteInvocation
ImportError: No module named psycopg2

in the logs of openerp-server:
[2010-07-31 13:30:24,874] ERROR:web-services:[02]: Environment Information :
[2010-07-31 13:30:24,875] ERROR:web-services:[03]: System : Linux-2.6.33.3-atom32-11052010-1-i686-with-debian-5.0.5
[2010-07-31 13:30:24,875] ERROR:web-services:[04]: OS Name : posix
[2010-07-31 13:30:24,875] ERROR:web-services:[05]: Distributor ID: Debian
[2010-07-31 13:30:24,876] ERROR:web-services:[06]: Description: Debian GNU/Linux 5.0.5 (lenny)
[2010-07-31 13:30:24,876] ERROR:web-services:[07]: Release: 5.0.5
[2010-07-31 13:30:24,877] ERROR:web-services:[08]: Codename: lenny
[2010-07-31 13:30:24,877] ERROR:web-services:[09]: Operating System Release : 2.6.33.3-atom32-11052010-1
[2010-07-31 13:30:24,877] ERROR:web-services:[10]: Operating System Version : #2 SMP Tue May 11 11:35:00 UTC 2010
[2010-07-31 13:30:24,878] ERROR:web-services:[11]: Operating System Architecture : 32bit
[2010-07-31 13:30:24,878] ERROR:web-services:[12]: Operating System Locale : NOT SET
[2010-07-31 13:30:24,879] ERROR:web-services:[13]: Python Version : 2.6.5+
[2010-07-31 13:30:24,879] ERROR:web-services:[14]: OpenERP-Server Version : 5.0.11
[2010-07-31 13:30:24,880] ERROR:web-services:[15]: Last revision No. & ID :
[2010-07-31 13:30:24,880] ERROR:web-services:[16]: Traceback (most recent call last):
[2010-07-31 13:30:24,880] ERROR:web-services:[17]: File "/usr/lib/openerp-server/osv/osv.py", line 58, in wrapper
[2010-07-31 13:30:24,881] ERROR:web-services:[18]: return f(self, dbname, *args, **kwargs)
[2010-07-31 13:30:24,881] ERROR:web-services:[19]: File "/usr/lib/openerp-server/osv/osv.py", line 119, in execute
[2010-07-31 13:30:24,882] ERROR:web-services:[20]: res = pool.execute_cr(cr, uid, obj, method, *args, **kw)
[2010-07-31 13:30:24,882] ERROR:web-services:[21]: File "/usr/lib/openerp-server/osv/osv.py", line 111, in execute_cr
[2010-07-31 13:30:24,882] ERROR:web-services:[22]: return getattr(object, method)(cr, uid, *args, **kw)
[2010-07-31 13:30:24,883] ERROR:web-services:[23]: File "/usr/lib/openerp-server/addons/project/project.py", line 176, in duplicate_template
[2010-07-31 13:30:24,883] ERROR:web-services:[24]: self.copy(cr, uid, id, default=default)
[2010-07-31 13:30:24,884] ERROR:web-services:[25]: File "/usr/lib/openerp-server/addons/project/project.py", line 170, in copy
[2010-07-31 13:30:24,884] ERROR:web-services:[26]: cr.execute('update project_task set active=True where project_id in %s', (tuple(ids,)))
[2010-07-31 13:30:24,885] ERROR:web-services:[27]: File "/usr/lib/openerp-server/sql_db.py", line 77, in wrapper
[2010-07-31 13:30:24,885] ERROR:web-services:[28]: return f(self, *args, **kwargs)
[2010-07-31 13:30:24,885] ERROR:web-services:[29]: File "/usr/lib/openerp-server/sql_db.py", line 122, in execute
[2010-07-31 13:30:24,886] ERROR:web-services:[30]: res = self._obj.execute(query, params)
[2010-07-31 13:30:24,886] ERROR:web-services:[31]: ProgrammingError: syntax error at or near "31"
[2010-07-31 13:30:24,887] ERROR:web-services:[32]: LINE 1: update project_task set active=True where project_id in 31

I can perform the same step with the GTK-client.
I have successfully checked to have python-psycopg2 installed (from the original debian repos).

What now?