Trouble with Uploading and Displaying Images

Question

Hmm, been doing WP development for a good long time, but this is a new problem I've just encountered and don't know the next step to track it down. The symptoms feel related but may not be. They are:

1. I can upload images from Media>Add New - though it appears to get stuck on "Crunching" despite adding to media library
2. If I attempt to upload an image through Gutenberg block>Select Image>Media Library>drag-and-drop or Gutenberg block>Select Image>Media Library>Upload Image, the image progress bar goes all the way across, waits several seconds, and then I get a message "An error occurred in the upload. Please try again later."
3. If I attempt to drag an image into a Gutenberg block or use Upload/Select, the image appears for a second and then disappears and a message says "The response is not a valid JSON response." Interestingly, it looks like the image is added to the media library anyway.
4. If I attempt to add an image to a CPT or to a Gutenberg Image block, using Select Image>Media Library the loading symbol spins and spins and no images display.
5. A CF7 form that was recently successfully allowing non-members to upload images, giving them a confirmation message afterward, now uploads the image but gets stuck on the spinning loading symbol.

No major changes or plugins were added since this problem developed. Though I know how little things can cascade into bigger problems.

Naturally, I've searched on all of these problems and not been able to conclusively see a common thread. I've searched the logs but nothing jumped out at me.

This is a production website, so disabling all the plugins and other such Draconian suggestions, are not easy without cloning the website to a safe place.

Here's the site: https://unavoidabledisaster.com

Any suggestions you may have may be helpful.

EDIT 1: Image sizes are set correctly in settings. After an image upload, I see the properly resized images in my uploads directory.