Strange little issue over here. I am writing this to spare you from losing the hours I just did. It is not related to any specific Garmin device.
In short, since a couple of weeks I am using a block list with over 400 URLs on my router (FRITZ!Box 7490). It saves me from the advertising I get on my mobile devices without having to use blockers. Most importantly, from the abject Outbrain and Taboola crap on my phone.
The thing is, somehow Spotify needs some of the sites that are listed because it won't synchronize otherwise. The setting up on Spotify on Garmin works fine but the syncing remains stuck at 0% until I get a "Sync Failed" message. To get it to work I need first to empty my blocklist on my router before syncing. Note that Spotify plays fine on my other devices otherwise, even with my blocklist activated. This is only related to syncing on Garmin devices. I have a similar problem using Netflix on a smart TV, not playing content when my blocklist is turned on.
If anyone knows what are the URLs required for Spotify to synchronize on Garmin, please let me know. I will then take them out of my blocklist.
Hope this is useful for someone out there
Cheers
Z.