Symptoms

When launching the SmartObject Service Tester tool, the following error was thrown:

Open Failed: Open may only be called on a primary login
Source:
SourceCode.Hosting.Client.BaseAPI.SocketBaseAPIConnection.Open

Error: The process cannot access the file 'K2ServerSettings.set' because it is being used by another process.
 

Diagnoses

It was determined that the SmartObject Service Tester tool was being launched from a separate client tool machine, in which the 'Tools > Server Properties' configuration was currently 'localhost'. As such, it was likely attempting to connect to a non-existent Blackpearl server.
 

Resolution

Changing the 'Server Name' configuration to the actual K2 farm name/hostname resolved the error.