There is a slightly longer message written to the error log too. I'm a bit out of my depth with session locking but the id may help to identify the root of the problem.
if ($this->time() > $startlocktime + $this->acquiretimeout) {
// This is a fatal error, better inform users.
// It should not happen very often - all pages that need long time to execute
// should close session immediately after access control checks.
error_log('Cannot obtain session lock for sid: '.$id.' within '.$this->acquiretimeout.
'. It is likely another page has a long session lock, or the session lock was never released.');
throw new exception("Unable to obtain session lock");
}