Edited to reflect only a partial fix:

I'm still amazed and disappointed at the apparent apathy to fixing the issue in any sort of hurry. It made me unable to play my saved music collection for over a month via my receiver.

Nonetheless, yesterday's update finally partially fixed the issue.

The same errors for tracks not being available are still appearing when trying to play most of what's in Recently Played. It's possibly a hangover of corrupted stuff from the previous update and I imagine any new items added to the recent list after the update won't have the problem. Anything I'm playing via albums or artists views is playing ok again.

If you're still having issues outside of this problem it may be worth toggling syncing of your Google Music account in Google settings. This has fixed some problems in the past with tracks not playing after an update.

It appears that after over a month, the developers have only fixed some of the issue. Albeit, the main way I play music (picking an album) has been fixed and I can get any track to play if I select Album view and try and play a whole album or an individiaul song. Custom playlists appear to play too.

Recently added/played still does not work. An album that appeared in it since updating still claims it can't find the track(s). Also, if I select an album or song via Artist view I get a lot of errors that the track can't be played. As well as this, Artist view is showing every album released by the Artist, not what I have in my own collection. I guess it's accidentally showing a subscription view.

Is there anybody internally working on this software at this stage? I don't understand how the last two updates could get past qc. Five or six weeks to only partially address bugs that rendered the entire software unusable for most who aren't on a subscription. I'm assuming that the dev team either doesn't use the product or is automatically entitled to subscriptions, meaning they never see the product from the pov of somebody who doesn't subscribe. Design 101 is to test and retest your product, utility, service or communication from every angle. I thought Google was a tough gig to land a development job with but I don't know if that still holds true if software like this is a reflection of who's developing and who's ok'ing it for release.