how to get a more significant error response from ServerSideRender

I’m having a heck of a time with the ServerSideRender component for a Gutenberg block I’m developing. I’m seeing this error in the (non-)rendered block: Error loading block: The response is not a valid JSON response.
But where can I see the actual JSON response?

In the Chrome Inspector Tools I see:

Failed to load resource: the server responded with a status of 502 ()

along with the request that was sent, with all the block attributes:

All the attributes look correct, and how they’re handled in the PHP render callback is looking correct. I had the ServerSideRender working until yesterday, and now suddenly this “non valid JSON response error” is popping up.

I haven’t made any changes to my render callback in PHP. I have even stepped through the render callback PHP function to make sure it was receiving all the attributes correctly and handling them correctly, and I have verified that that is the case. I have a function that will write to a debug.txt file, and I have stepped through the render callback making sure that each condition is working correctly. I can even see the final output that the render callback is returning, and it is correct. And yet, I am seeing this error on the Gutenberg block side. It would be helpful to have a more significant error message rather than “not a valid JSON response”.

Looking at the Network tab of the Chrome Inspector Tools I am seeing that the actual response from the request posted above is 502 Bad Gateway - nginx.

I have no idea what this is supposed to mean, because the request from the block is being sent to the server callback, the server callback is processing the request and returning a correct response. Something is happening somewhere between the response that is returned from the callback and the client side rendering of the block. Any ideas what could possibly be going on?