I get that too, when uploading from xls - however the workaround I use is:
After uploading the xls I then go into form builder, hit edit, change one question (e.g. add a space) then save and change the question back to what it was before and save.
After that I can preview the form just fine, even though the form itself (structure and content/labels) is exactly the same as what I had uploaded via xls.
Would love to know what the reason for this preview error is - I’ve seen it when previewing other peoples’ forms as well so I don’t think it’s just me doing something wrong!
