The set session value server behavior can be installed from the Cookies Toolkit or eCart. If you have a newer version of eCart you should try installing it to see if that clears up the issue. If you do not have this or the issue persists you may need to try the following.
Delete the cache file
If this does not work then there might be a corruption of your configuration folder, if this is the case you will need to follow the steps here to undo this:
If the issue persists after trying these things please let us know.