How and when did you create the token table? If I delete and re-create it, it works again.

This commit is to blame: https://github.com/LimeSurvey/LimeSurvey/commit/f5606a3ac2a0f72dd0f0470ad14ddf1031d76b51

But it's not possible to revert it because
1) It's too big
2) I've refactored files after, so there are too many conflicts.

Olle, the token table was created during survey creation.

But even if I de-activate the survey, then activate again and during activation decide to use tokens and then loading the old, backed up token table, I can still enter the survey without being asked for a token.

Thus, I assume the token check somehow fails. Maybe it checks if the not existing token is empty...?
Something like "..WHERE token='$token'" with $token being empty AND the token table containing empty tokens?

Actually this only seems to happen if none of the participants have a token code.

After closer examination i could reproduce the bug.
The bug ONLY occurs when there is a token table and all the tokens are empty.
It is due to an incomplete comparison, that only checks if the token exists in the database.
Though token is empty and clienttoken (The POST/SESSION-token) is also empty they match, leading to a seemingly unsecured survey.
If there are tokenstrings inside the token table the problem is not occuring.
The incomplete comparison is fixed.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=20420

Version 2.51.4 released.