I've used sonos for absolutely years and never had a problem accessing my music library.
Nothing has changed on my network. I've removed my music source and tried to re add it, but it doesn't matter whether I try and add the network share, or a direct local path, I get the same error saying
Please check the path to the folder, and the username and password if needed are correct.
Reason: The computer 'fileserver' is not responding.
There is absolutely no problem accessing the server from another machine. The share and password works absolutely fine - the same as they have for years and years.
I've reset the whole sonos system.. I current have just one wired connect device to the network for trouble shooting... still not joy.