Hi @trinityimma 
Unfortunately, there could be many reasons for a callback failure…
My first guess would be that there’s something wrong in the code that made the server shut down.
You can get a lot more information about error messages if you expand the message by simply clicking on it.
You could probably get more info as well on your terminal (or command line) from which you launched your app.
It would also really help if you could share your code.
But if you don’t want to share it, maybe you can start with sharing more info on the error messages 