Can you share a screenshot of what they are seeing? Also can you view their partial response and see what is being logged? An export of the survey would also be helpful. Without those, it's hard to know what the issue might be.

Agree with MattBroffman_Orlando . Also, I've found that respondents will sometimes interpret the error message to apply to the question above where it appears when it's really triggered by the question below. So be sure to rule out that possibility.

Thank you!
I have asked one of the participants who mentioned the issue to take a screenshot of the error message, but apparently she does not receive an error message but simply cannot continue by pressing the "next" arrow. The data are logged (partially). I can see that on the page where the error occurs no data are logged though. So data logging ends on the page before (which is not much as it happens in the beginning of the survey). In attach a Word and .qsf of the survey (sorry, it's all in Dutch). The issue occurs after block "Demografische gegevens" (Q17). I also asked the participant to double check if all items were completed, and she has confirmed that that is the case, but she still cannot continue to the next block.
De_invloed_van_COVID-19_op_cognitief_functioneren (2).docxDe_invloed_van_COVID-19_op_cognitief_functioneren.qsf.txt