I'm sorry to hear that you are having difficulties with Showkase.
Unfortunately, this is not an error that I've encountered before (wither personally or via a user report) so, at the moment, I do not know what might be causing it.
Has anything changed on your web server (e.g. PHP version) recently which might somehow be contributing to your problem?
What version of PHP does your web server use? Do you have the option of updating it? If so, it might be worth trying. (The current stable version is PHP 7.4.12.)
Before trying anything, please make a full backup of your current site, just in case anything further goes wrong and you need to reinstate your files at a later date.
It sounds like the error is a server error (rather than specifically a Showkase error) but I'd try running Showkase in debug mode in case any more descriptive error messages are displayed in the Showkase status bar before the JSON error occurs.
Open your 'showkase/admin/settings/constants.php' file in a plain text editor and change line 12 from:
Please note that the line number above refers to the current version of Showkase (v1.7.5).
If you have recently upgraded to Showkase v1.7.5, then I'd also try going through the upgrade process again (with the same v1.7.5 files) to ensure that all Showkase core files are present and correct on your web server (in case anything went wrong during the initial upgrade process resulting in corrupt or missing files).
One more thing I'd try is to go to 'System -> Repairs -> Repair' (in Showkase's interface) to rebuilt the page records.
If none of the suggestions above work, then the best course of action would be to determine the exact nature of the problem.
If you can access your web server error logs, then there should be an entry corresponding to the JSON error which might hopefully let us know exactly what Showkase file (and what line of code within the file) is causing the error.
Please let me know what your error log records for the problem.
I hope these notes help.
Please let me know how you get on and if I can be of any further assistance.
I'd be happy to help further might it would be helpful if I had access to your web server and Showkase site so that I can reproduce the error for myself and investigate further.
If you continue to experience difficulties and are agreeable to allowing me access, then please email me (my email address is below my username to the left of this post) with your FTP and Showkase login details. (I believe that I have previously logged into your web site a while ago.)