From MoodleDocs

The most likely reason for obtaining an "Invalid Sesskey" message is because your session has timed out i.e. you were inactive for an extended period. Please log in again.

Other possible reasons are:

  1. Your site doesn't have the PHP setting "session.auto_start=0" (see Installing Moodle - software)
  2. You logged out at some time between opening the form and submitting it.
  3. You have followed a bad link to a form script.
  4. Your database has an incorrect value for sessioncookiepath in the mdl_config table.
  5. You edited the code in a file on the server while logged in to your moodle website.