ACS internal server error

I recently rolled back some code changes in git to a known functioning point. Now when the app executes an ACS query, all I get is

[INFO] :   Error:
[ERROR] :  : Internal Server Error

even though I have error message output set to display like this:

Ti.API.info('Error:\n' + ((e.error && e.message) || JSON.stringify(e)));

That's not enough info for me to start troubleshooting. I double-checked all the API keys for correctness. I can perform the query on the web side of the app. I can't set breakpoints on the query. So what else can I check?