Although I haven't been able to make the error occur, a couple of my colleagues have sent me a screenshot of the error which reads, "Cannot read property 'add' of undefined." What is this referring to? I have gone through my survey questions to find what could be the issue compared to some similar errors. However, I haven't found a solution for 'add' as the key word.
This error in the web app is usually related to choice lists or the use of unsupported expressions or pulldata calculations.
Taking a look at your xslx file, did you create this survey originally in web designer, and then download it in Connect?
What changes did you make in Connect and what version are you using?
I just tested your xlsx file publishing from 3.12 release and 3.13 beta Connect, and when I open in either 3.12 or 3.13 beta I do not see that error.
When you check the version locking in the setting for the survey in the website, what version is currently being used when the survey is opened in the web app?
If it happens again, can you confirm what browser they are using, the version of the browser, and also on what OS version.