I've had two instances of the same bug today. The symptom is this:
You use Context, the Syntax CMS administration tool, and it works fine except that once you login, any link you click on will take you back to the login screen. It just can't remember your login.
That's because it can't write the session files it needs. The solution is twofold: Make sure the directory private/tmp/session/ exists and is readable and writeable by the Web server. Then, in your public/.htaccess file, make sure that this line exists and points to your site:
php_value session.save_path "/path/to/your/Syntax/CMS/instance/private/tmp/session"
Once you do both of those, your admin tool should work (or at least be free of that problem).