This could be a regional server issue that only affects some users.
FYI for anyone happen to be experiencing a similar issue.
To debug this, you need to use a web browser (Note: I use Firefox) on a computer or smartphone. Open the Jazz Groove account settings page and sign in to your Premium account, go to Audio Setting. Choose the FLAC quality, a station you’re having problem with, and Stream URL (not PLS). Copy it to a new browser tab and play it using the browser. If you can reproduce the problematic playback with the computer or smartphone, copy the URL from the browser (it is no longer the same as the one on the Jazz Groove account page because it has already redirected you to a new regional server - you need the final link) to an e-mail.
Once you’ve got the actual final (already redirected) link(s), you can send these links to Jazz Groove support, they’ll forward it to the streaming service provider and fix the problematic regional server.