This issue is occurring on Craft 2.9.2.
The assets are stored on Amazon AWS (S3).
When I try to uploaded a new asset (of any size, even very small), the progress bar moves to 100% position but remains on the screen, and the loader sprite in top right corner keeps going around. The same symptoms are described here. The post was of no help. There was an answer in that post, which links to here. Which was also of no help, as the suggested nginx.conf setting is already in place, and the php settings are more than adequate.
Nothing shows up in the craft error log.
Nothing shows up in the server (php) error log.
Can anyone suggest how I might go about troubleshooting this, or suggestions as to what the cause might be?