The king of all "error occurred in the upload" glitches…

My apologies for adding yet another support request of this type, but I’m an extremely seasoned web designer/developer and I’ve already tried everything I could find on this issue. As with many others, my problem is that I can’t upload any images, I keep getting this:
“An error occurred in the upload. Please try again later.”

It should be noted that it is actually uploading the images in question – I see them in FTP every time – it just doesn’t seem to be able to register them in the Media Library. Here’s what I’ve tried/looked at:

– Reverted to the Twenty Sixteen theme.

– Disabled all plugins.

– Reinstalled WP both via the Dashboard button and by re-uploading through FTP.

– Re-exported the images (note that they’re not large – again, I’m a professional designer) from Photoshop and used new, unique names.

– Futzed with folder permissions, making ‘2016’ and ‘5’ (this month) set to 775 (World can’t write, but everything else allowed).

– Tried two different “add from server” type plugins, both claimed to be adding the uploaded images, but neither actually did so.

Note that this site is running on my VPS, on which MANY other WP sites are installed and working just fine. However, one possible issue is the fact that this site doesn’t yet have its real domain name – it’s using an IP address + username:
https://96.127.170.113/~greycourt

The site was installed with that in mind, the above is reflected in the General Settings and everything else is functioning correctly, so I’m a bit skeptical that’s the problem, but I don’t have any other ideas right now. Anybody have any idea what I could look at next?

Thanks!