I get the following message: “You need EssentialPIM Pro Business license to connect to EPIM server” when trying to open a database file over network.

It means you’ve exceeded the allowed by your EssentialPIM Pro Business license number of simultaneously connected users to the database. The Business version requires a separate license for each of the connecting users. In other words, the number of licenses should equal the number of user installations. I.e. if you have 10 installations, you need to purchase 10 licenses. Note, only EPIM installations need license, server can run without EPIM installation and thus doesn’t require an additional license.