Never had an issue with this plugin with the exception when I installed it and did wrong one step. I myself help others with similar issue long time ago. Never an issue with Spotify since then, including all the OBS and Tuna updates.
Clearly, you guys had a problem and wanted a magical solution. If no one else can reproduce the issue you're facing, how come you can fix it?
Too many variables and differences on each computer can lead to unexpected results.
The redirect URI is the one you have to set in the Spotify Devs site and points to
https://univrsal.github.io/auth/token (you can create your own if you wish) as it's shown on the video where it's explained how to do the spotify devs thing, and it's accesible in the Overview tab under the How do I display the information? / Spotify
And yes, this was the issue I had when installed Tuna years ago.
By the way, in order to be able to edit your posts you need to be older in the forum and post more often. Been helpfull.