The support questions seemed interested in the account's address. I wonder if that address was in a restricted country, known mail forwarding vendor, or PO Box?

To be clear, I think the way this was handled and tech company's whole "we cannot tell you why" stuff is unacceptable. But typically we can make a fairly reasonable guess why they might restrict the account, we just need more information (e.g. VPN? What was the item? Is your address unusual? PayPal account's history?).