Export Subscribers just spins

This has been an issue we have had for some time. When we attempt to export subscribers (any number, any fields) when you click “export” the next page opens, provides the message, “Processing export, this may take a while. Please wait” and the little spins, and spins, and spins…never to return.
We have upgraded several times, done several clean installs and no luck. Any suggestions?

I have seen others are having problems with the export function, but not exactly the same as ours.
This thread https://forums.phplist.com/viewtopic.php?f=35&t=41801
from 2014 has a solution, but their issue was the export finished, and they couldn’t see the results. Our export never finishes, the logo just keeps spinning.
Any ideas on where to start looking?