Previously, all signing links generated by Box Sign were on the app.box.com subdomain and not on the subdomain configured via the BVE EID.
So, if a customer on a Box Verified Enterprise EID blocked app.box.com as part of their network settings, their internal users couldn't access URLs that Box Sign generates, and hence can't complete the signing process. More info here.
We have released Box Sign support for Box Verified Enterprise (BVE) EIDs. Stay tuned!