Thanks a lot for your super-quick replies. I really appreciate the feedback!

holch wrote:

3) still has values for the same token in an earlier backup of the exported data

With your setting, this says very little, because you can have more than one response from the same token, as you allow multiple response with the same token.

Together with the "token-based response persistence" enabled this setting makes re-using tokens open the filled out survey submitted (or even only partially filled) earlier and not open a new blank survey, correct?
My point is that there is no way way for a user to "un-select" radio-questions where a selection has been made and submitted before (please refer to the attached image).

It's probably possible to manipulate those questions using URL-parameters (something like "&qcode=NA" at the end of the survey link) but that's definitely not what has happened in those instances.

jelo wrote:

mos3465 wrote: Any suggestions on how to progress?

Open a bugticket, even with an old version. You will receive the answer that your version is old, but you might get the answer that there already is a bugticket for that and it was solved or not.

Did just that, adding the link here for future googlers' reference:
bugs.limesurvey.org/view.php?id=15621

My point is that there is no way way for a user to "un-select" radio-questions where a selection has been made and submitted before (please refer to the attached image).

Probably not, if it has been submited (click on a button to send the page, e.g. next or submit).

Yeah, that was the case. Several days between such a value being submitted and vanishing again.
Let's see whether the bug report yields anything. Will update this thread if it does.