Today I learned, that you can't use a dot in the string you feed the session_name() function.
From the documentation:
"The session name references the name of the session, which is used in cookies and URLs (e.g. PHPSESSID). It should contain only alphanumeric characters; it should be short and descriptive (i.e. for users with enabled cookie warnings). If name is specified, the name of the current session is changed to its value."
The session name can't consist of digits only, at least one letter must be present. Otherwise a new session id is generated every time."