Error when saving UserPreferences

I have a Problem with the UserPreferences. An existing user can login. After
the login there ist the username in the right top corner.
But it is not possible to change to another page without loosing the login
(there is written UserPreferences in the right top corner again).
And I can not save changed preferences. Then a message shows up wich tells
me that the user already exists (ofcourse it does...) and the
user is not logged in any more.

I've already checkt the following:
* cookies in the browser are enabled
* wiki-user have the rights read and write for the files with the
preferences (data/user/<id>)

Lately we moved the wikis from one server to another and now use a newer
python version.
It used to work fine on the old machines.

Thanks for help.
Grit Hieke


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