The URL for phpMyAdmin that we are using has a port number, and I suspect that the port number is not being anticipated or read properly by some code somewhere, because when we click Apply on any of the Settings pages (ie: Features, SQL Queries, etc), the page reloads and the URL starts with simply: https://:8443/
The settings are saved, so this appears to just be a problem with where/how the current URL is being detected to act as the action of the form, or some other redirection as part of the ajax submission.
--Julian
Cannot reproduce your problem. I tried with port 81, both with and without defining $cfg['PmaAbsoluteUri'].
Which PHP version?