You’ve exceeded amount of simultaneously connected users to database. Regular EPIM Pro version allows one user connection over the network. For more connections you need network license for each connected user. The number of licenses should equal the number of user installations in your EssentialPIM Network. 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.
If you are sure that the amount of connected users is correct then most probably one of PCs has two EPIM clients running. Login as SYSDBA then navigate to Tools -> Users -> Connected users. This tab shows what users are connected currently. Check which user is connected twice and close exceeding EPIM client on corresponding PC.