Hi,
by taking a look at the content of the response message (besides its status code) you can have a clear idea of the error cause.
In your example, given that the language is not made explicit via the "lang" parameter and the two texts are extremely short (actually just two words), the language identification fails to identify the two words as English. This could happen sometimes when the input texts are just words or extremely short, and that is also the reason why you do not always get the error.
If you know that your texts are in English, I suggest you specify that through the "lang" parameter.
Cheers,
Roberto Santoro
Dandelion API team