Hey @nebulon thanks for letting us know you can reproduce the issue. I will keep an eye on this thread but probably move away from using Koel for the time being.
Again, thanks for all your help and time!
OK, found the root cause. The config.php contained a spurious 0 => 'NANphp',. When checking the presence of the redis.cluster property in the config, the in_array function can return true when the config contains a 0 property as (int)'redis.cluster' === 0.
https://github.com/nextcloud/server/pull/29906 is the upstream PR.
Solution: remove the spurious 0 => 'NANphp', in config.php using the file manager .