The owner and permissions of the data/sessions folder and the user group that moodle is running as. Moodle as a php program is usally running as apache, since I believe, php runs a sort of subprocess of the apache web server. The data/sessions directories should be owned by apache, or by nobody, or the permissions should be set to a high enough level so that moodle can change them.
Alternatively you can trying setting dbsessions to on, so that the session data is saved in the database.
Tim
Hardware and performance
Troubles
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