One thing that saved me when i was having these kind of problems was using the ProjectAll option from the Subtool menu.Basically,i export the lowest subdiv model,unwrap it(in UV Layout),than i go into zbrush,import the unwrapped model,subdivide it as many times as the high poly version is subdivided,than i add the high poly version as a subtool(with all the lowest subdiv levels deleted) and hit project all.All i can say is that it worked for me everytime,making me wonder why zbrush does not have any errors importing/exporting objs when doing this.At least on my PC it doesnt 