Cannot save settings

I've posted about this before, but still have been unable to resolve it.

When attempting to save settings on my installation, I get the spinner that shows it is saving, which stays there indefinately, and on refreshing the page, the settings are not saved.

However, another user on my cpanel vps (iamkangy on this forum) has their own install for their site, and saving settings works on theirs.

Previously, the only differences between the installs were the php versions, but I have now set my install to use the same php version (I have multiphp selector installed in cpanel), and have created a subdomain for my install, which I have set to use php 7.2 (the same as iamkangy).

When clean installing, I completely emptied the database and folder, and uploaded the latest version of the script, so i'm at a complete loss as to what is happening.

There are no errors logged at all in either the logs folder, or the server's error logs.

If need be I would be willing to give access to my install, and also cpanel access if you require it to find out why this is happening.