I would need more information and you will find below some instructions, assuming you are running Polyphone on Windows.
I suspect a wrong configuration to be saved somewhere so could you please open the registry (write regedit in the Windows menu) and go to "HKEY_CURRENT_USER/SOFTWARE/polyphone/", then export the full content of this node and share it so that I can investigate (in a zip file)?
Then you can open a terminal in the installation directory of Polyphone and write the following so that the configuration is cleared:
Open polyphone and test the keyboard with an instrument... is it now working?
If you cannot export a part of the registry you could still tell me if, after having run the command, polyphone is now working correctly.