Why does the administrator logoff all of the sudden

From Joomla! Documentation

In Joomla! 1.6, the session table data column was changed from a TEXT datatype to a varchar(20480).

That is not enough storage capacity for the needs of many Components. In cases where the memory needs are exceeded, a silent error results that manifests as a logoff to users.

To fix, use a tool like phpMyAdmin to change the datatype of the data field from varchar(20480) to MEDIUMTEXT or TEXT (either is fine.)

This issue will likely not be fixed until version 1.7 due to challenges with changing the SQL Table definitions for upgrades. For now, manual changes are required.