Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page

I couldn't find an answer to this in other relevant threads.

I'm trying to upload a PDF (1.4 MB) to WordPress using the media uploader and I get this response:

Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page.

This is on shared linux hosting at Godaddy.

What I've tried:

• Disable each plugin individually

• Change to wordpress twentytwenty theme

• Raised config settings in user.ini

  max_execution_time upload_max_filesize post_max_size max_input_vars memory_limit client_max_body_size

• Log in incognito mode

• Clean install of wordpress on live server

• Changed wp-content and uploads (+ subdirectories) file permissions to 755

• Made async-upload.php DOING_AJAX without if statements

The error log shows nothing.

In the server logs I have found a 404 for async-upload.php, which I suspect is the root of the problem, but I don't know what to do with it. (async-upload.php file is definately there).

Does anyone have any advice on this?

Thanks

It was a corrupted PDF. The first thing I should have checked.