undefined extension class [SOLVED]

I've tried to replicate the problem you reported (by publishing after sorting a ListViewer gallery by 'sort newest first', as seen in your screenshot) but I'm not encountering the error message myself.

First of all, you could try reinstalling Showkase v1.7.6 on top of your existing v1.7.6 installation .
Follow the upgrading instructions (https://showkase.net/support/upgrade/) but with the same version that you are currently using.
This will overwrite all Showkase source files with fresh versions, in case any of your existing source files have somehow become corrupt or gone missing.

Important Note:
You should not lose any custom content (pages or settings) doing this but it would be wise to make a complete backup of your existing site first, just in case anything unexpected should happen and you need to reinstate your original files at a later date.
Also, if you have made manual customizations to any Showkase source files or have used your theme's custom.css or custom.js files, then please note that these will be overwritten when you follow the upgrade process and you'll need a backup in order to reinstate them afterwards.

If this does not help, then in order to try to determine exactly which line of source code is triggering the error message, try running Showkase in debug mode (in case any more descriptive error messages are displayed in the Showkase status bar).
Open your 'showkase/admin/settings/constants.php' file in a plain text editor and change line 12 from:

define('DEBUG', false);

... to:

define('DEBUG', true);

The line number above refers to the current version of Showkase (v1.7.6).

Please let me know what new error messages (if any) are displayed in the Showkase status bar when you try to replicate the problem.

Also, please let me know what version of PHP you are running (in case it is relevant). I have been testing under PHP 8.1.21.

Hopefully this will resolve your problem or at least bring us one step closer to figuring it out.