I am using the main bootstrap template on my survey and it is throwing that error.

Update: same happens with the unmodified vanilla theme too

Can you attach an exported survey containing only that question?

How can I do that? There is quite a lot of confidential data in the survey already.

I can export the question group if you tell me how

That's why I said "containing only that question". You will need to make a copy.

Made a copy, see attached

That survey includes 3 groups and several mandatory questions. Sorry, but I don't have time to test unnecessarily lengthy surveys - that's why I asked for a survey containing only that question.

sorry, see attached

It works for me when isolated. There may be something in another question or your theme interfering. Try placing that question in a group by itself.

By the way, I assume that you have updated to the latest stable version.

I isolated the question and made a new date question in its own group, but it does the same.

Current version is: Version 3.3.0+180209

I'm sorry, I've run out of suggestions.