I’m guessing that a non-Lemmy server doesn’t provide some optional fields that Lemmy servers always provide, or that it provides some unexpected but valid data points in the API. The API docs aren’t exactly very detailed, so I wouldn’t be surprised if LJD assumed some optional field to always be present (because on every post on Lemmy that has been the case).

I took a look at the JSON Lemmy generates and it seems perfectly fine. Unfortunately, Sync is obfuscated so I can’t get a clue at what object is malformed in the database to see what could be causing this.