Assuming you haven't renamed machines (which brought me great pain ! even after sp_dropserver / sp_addserver etc)

I had to enable SQL browser on my SQL express (subscriber) instance, which then mediated nicely so that I could use the <hostname>\sqlexpress target - now this works.

Previously I had a default config of express, which did allow connection via SSMS from the publisher machine to the <hostname> (ie: no \sqlexpress) ...but would throw the '...actual server name to make a connection to the server..' error when I tried to setup replication.