Despite the launch of YouTube's streaming service, many music fans remain attached to Google Play Music because of one feature: the option to upload up to 50,000 songs from your own collection and listen to them on any device. Over the past days, however, several users have complained about their inability to do just that because the desktop app needed to upload songs, Music Manager, isn't connecting to Google Play.
Most users are facing the same error when they try to log into Music Manager: "Couldn't connect to Google Play. Check your internet connection and try again." No matter how many times they retry, from different computers, they still get the same error. Even the up, up, down, down, left, right, left, right, b, a workaround mentioned here doesn't work. The page it redirects to in Chrome doesn't load properly. I had the same experience on my Mac.
The reports seem to have started coming in around mid November, but have intensified in the past few days. If we were to put on our conspiracy theory hats, we'd say that this might be Google's "subtle" way to force everyone out of Play Music and into YouTube Music, by taking away the feature they love most about the former. It's more likely though an issue of neglect and ignorance. If the service is all but abandoned, it's possible that the few people still maintaining it haven't noticed this problem yet or haven't gotten around to fixing it.
If Music Manager isn't working for you, you can open Play Music in the browser or through this Chrome extension, then drag-and-drop your songs there. Beware though, this method is slow and some users are also facing issues with it.
Music Manager appears to be working again on desktops. Several users have reported this on the different support threads linked below. I also verified that it's back for me on my iMac. Thanks, Christian!