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Your LimeSurvey version: Version 5.6.53
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Hi everyone,
we have embedded two surveys via iFrame on two websites. We are only using LimeSurvey Cloud and don't use an API. 

The first one works great but the second is showing the "CSRF token could not be verified" error to some users when they try to submit the survey. 

We don't know how this problem occurs since both surveys are implemented in the same manner. The only difference is that the second survey has the same ID for both domains since it makes it easier to evaluate. 
We use alias domains and the iFrame embedding is enabled. 

Does somebody have any idea how to solve this? 
 

The system need server update about session samesite (at minima)

manual.limesurvey.org/index.php?title=Op....28New_in_3.24.3_.29

You need to contact LimeSurvey GMBH

Even with this settings : the original server can disallow any external iframe !

See : developer.mozilla.org/en-US/docs/Web/HTTP/CSP