Named SQL Server that connection stopped to run if named connection

Question

I installed a named SQL Server Express edition engine, with SQLEXPRESS as its instance name, some months ago. An application used to connect to that engine with a named connection. Last week it stopped to work. The named connection (servername=ip\SQLEXPRESS) is no longer visible on network, even though it works fine locally. Connecting from the network works fine using an anonymous connection and a static port (servername=ip;port=portNr). Why?