I'm no help but I was having the same issue and couldn't find anything in the settings that controlled those error messages. Annoying.

I'm also not sure why they're not formatted properly.

Usually a lemmy client expects a certain kind of response after fetching a post. If it's different than what it should be (like this error), it often just outputs whatever else came. In this case it's a raw JSON encoded string.

There are ways to avoid this, and to output a nicely formatted error but usually the effort it takes is not worth the benefit it provides.

Fwiw, the issue seems to have gone away in the latest beta version of the app