SuccessfulKoala55 That string was autogenerated by pyhocon and matches their documentation too - https://github.com/lightbend/config/blob/master/HOCON.md#substitutions
The first example won't work (it will treat ${...} as a string literal and won't replace it). The second does work, but as mentioned anyway, these were not hand typed, but rather generated from pyhocon, so I don't think that's the issue 🤔