Hi, we found a bug (maybe is T4 framework not this template only) with another extension and support give this answer.
Hello,
the frontend error was actually caused by the T4 Template Framework system plugin "T4", apparently the file author.php is missing in /plugins/system/t4/src/t4/Helper
But yes, this error is only triggered by osgcourts on your site. Unluckily the former developer has used a common context "com_contens.article" by calling a plugin interface, which triggered the t4 system plugin, that tried to load an missing help class "author". And yes we have already corrected the plugin interface context in osgcourts to avoid such problem, I have already installed the fixes on your website. This fix will be included in our next release.
And yes, you can report the issue to your template provider, they should review the t4 plugin which they delivered along with the template, "author.php" is missing in /plugins/system/t4/src/t4/Helper, it can cause multiple potential issues.
Extension https://courts-reservation.ch/en/