When a candidate finishes the survey, the score is calculated with hidden equation questions after the last question.

This takes some time, since there are a number of hidden questions and a lot of calculations taking place.

Often - but not always they get the following error (in Dutch):

"TOT ONZE SPIJT IS UW SESSIE VERLOPEN.
Of u bent te lang inactief geweest, of u heeft cookies geweigerd in uw browser, of er waren problemen met uw verbinding.
For further information please contact:
xxx@xxx.xx"

All the information is received correctly, but candidates think the survey went wrong. It happens on different computers and with different browsers.

There are different surveys where this problem occurs, all having hidden score calculating questions at the end. I believe LimeSurvey 2.xx did not have this issue.