Try powering off all your Sonos devices then reboot the router and bring all back online as follows:

• Router
• Wired Sonos Products
• Wireless Sonos Products (nearest first)

Let each device settle and reconnect and then try saving a playlist again and see if the above manages to fix things for you. If not then immediately after the error occurs, it maybe best to submit a system diagnostic report and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can suggest to resolve the issue.

 A Sonos playlist or a music service playlist? Or are you trying to save it to My Sonos / Favorites?

I’m trying to save a third party playlist, Apple Music, as a Sonos playlist. I update the third-party Apple Music playlist when I’m listening to music through them, and then try to update the existing Sonos playlist, which is when I get the error message.

So let me get this right:

Empty Queue, load Apple Music Playlist (into Queue), go to Queue panel, Save, type name, get error message? Screenshot of error message please.