I was able to import one page - I know we had some technical issues around this same time that you were trying to import potentially, so I would also try again. I would start with maybe just the style guide import or just a wireframe import. I would not do the "Design" import because that imports both the style guide and a page, and that process can sometimes hang up depending on certain conditions - most of which are unfortunately out of our control.
But yeah, I would try simply importing a style guide first, instead of a design. (mine is greyed out because I just imported it).
Let me know where you land on this - keen to get you unblocked ASAP.