In the config.yaml, check the instance-languages line looking for a missing opening bracket…
It should follow this pattern instance-languages: ["nl", "en-gb", "fr"] or `instance-languages: [ ]
etc.
In the web admin config panel of gotosocial, the opening bracket before the list of languages must be left. If removed, the config file is no longer valid. And instance-languages indeed corresponds to the interpreted line 33…