Yup, very likely that.

My crystal ball says that your client is trying to parse the response of a failed request as it was a successful one.

A successful request would return a valid JSON object, while a failed on would probably return an error message.

Report a bug with your client devs saying that they should check that request's status code says it's successful before trying to parse its response as JSON.

lemmy itself is clearly trying to tell the user “hey, at least i’m not written in java”