Invalid index tag name 'Album Artist'

[zekeblue]

I've been getting this error (Invalid index tag name 'Album Artist') and my control point can't see MS as a result (I assume). NAS says it is running. Rescan always results in this error even after restarting the NAS.

Synology with latest DSM updates, BubbleUPnP on Android tablet, Cambridge Audio Azur 851N, all wired network except for control point on wifi.

I can see and play everything with the Synology native app.

I've looked at tags of more recently added music, but don't see anything suspicious.

Any ideas of where to look to troubleshoot this?

Thanks in advance!