It can be ignored. We're going to fix this in a future release.
When Flarum renders a page, it returns the single page app together with data that has been pre-loaded for faster rendering. This is achieved by making an internal request to the Flarum API. If the API returns an error, the Flarum application might then throw that warning/error because the code currently assumes that if JSON was returned, it would not be an error.
The real error that was encountered by the API should be logged as its own entry in the log file.
