Also, i don't understand why session-value does not return the saved string value but generates the "internal error", try this:

(defun f ()

(let ((username (hunchentoot:parameter "username"))

(session (or hunchentoot:*session* (hunchentoot:start-session))))

(if username

(setf (hunchentoot:session-value 'sessu session) username))

(format 'nil "session:~a value:~a~%"

session

(hunchentoot:session-value 'sessu session))))

First send the username:

http://127.0.0.1:5050/l?username=jose

after that, get the saved session value

http://127.0.0.1:5050/l