Initial survey settings:

• group by group (1 group = 1 page)
• "Enable token-based response persistance = No" and
• "Allow multiple responses or update responses with one token? = No"

Participant fill out survey form completely, so LimeSurvey sets "survey_sid.dataset.lastpage=-1".

Customer changes survey settings:

• "Enable token-based response persistance = Yes" and
• "Allow multiple responses or update responses with one token? = Yes"

Participants uses link for finished survey.

PHP error "undefinded index: gid"

Reason in file ".\application\helpers\expressions\em_manager_helper.php":
/**

• Check a group and all of the questions it contains
• @param <type> $groupSeq - the index-0 sequence number for this group

• @return <array> - detailed information about this group
  */
  function _ValidateGroup($groupSeq)
  {
  $LEM =& $this;

     if ($groupSeq < 0 || $groupSeq >= $LEM->numGroups) {
          return NULL;    // TODO - what is desired behavior?
      }
  ...

Function _ValidateGroup($groupSeq) is called with $groupSeq=database.survey_sid.dataset-lastpage=-1
The function returns NULL, that's it.