SIM900 throws error after resetting and re-connecting.

Sounds strange - The {"total_documents":0, ... stuff sounds like that's the JSON response to your request? If so, could it be that you're calling setTimeout, setInterval, or some function that expects a callback with that string as an argument?

Basically Espruino can often take a string or a function as an argument to callbacks - usually you'd just have something executable in that string so it's fine, but if you put something like JSON in it, it will fail. If you put "Hello World" in, that will work, because it's valid JavaScript, but {key:value} will fail as it's not valid (afaik surrounding it with brackets is valid: ({key:value}))

It'd be an idea to figure out what variable contains that data, and then go through your code and figure out where it is used.