I have been going through my ~/TODO list recently and I have meant to figure out why my Sonos indexing has been failing lately. I sync my iTunes Library from my Time Machine backups into a shared space on my NAS so other things can get to it without having to have my Mac on.
I tried to re-add the UNC path and it would consistently return error 900.
Google wasn't helpful at all on what error 900 actually meant.
So I cranked up debugging on samba and this came across:
No protocol supported !
I had recently disabled SMB1 on my NAS but didn't realize that change coincided with my indexing failures.
So tl;dr, it looks like Sonos uses SMB1 to connect to your NAS, so make sure that you leave it enabled.
Dear Sonos... please use a newer version of SMB... SMB1 is terrible.