It is possible to have a "modular" BIM server, which serves both ArchiCAD 13 and ArchiCAD 14 projects. (Even if you are planning to eventually migrate all your projects to 14 and abandon 13, the migration process is easier if you have a modular server). To have a modular server, you have to update the BIM Server 13 to Hotfix first, then install the BIM Server 14. The two should live in peace and harmony then on. Later, if you want to, you can (but don't have to) remove the 13 module - each module has a separate uninstaller in the "Uninstall" folder.
Some screenshots of the message - even in Italian - might help us understand the problem, or you can also contact your local tech support.
Gergely Kmethy
VP, Customer Success, Graphisoft