I had the same issue, reached out to Sonos helpdesk. Very helpful, they used remote access to fix it. Apparently my PC windows 10 was running SMB 1 security which has to be disabled and SMB 2 installed….. plus file sharing permissions had to be changed.

i tried to follow the process but it was beyond me!!  However it allowed the new app to access my iTunes music library!! Hope that gives some help……