It might be worthwhile to remove your Plex account from Sonos, then re-add it.  That might clear any old reference to the previous library location.  Although I believe access to plex works through the cloud (for Sonos access) so it may be an bad value in your account in the cloud or something like that. 

I recently added a 2nd library for my plex account and didn’t have any issues.  Not helpful, I know, and not exactly the same issue, but you should be able to do this.

I tried twice and then decided to remove it again and leave it off until tomorrow. 

Thanks