critical error when starting the /shop page

[jochriku]

When I start the shop site with grand-prix-tickets.com/shop there occurs a critical error:

When I debug the error, I get the following message:

Deprecated: Return type of SweetAlert2::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /homepages/26/d13363536/htdocs/clickandbuilds/TMTest/wp-content/plugins/wc-product-subtitle/vendor/varunsridharan/sweetalert2-php/sweetalert2.php on line 158

Please help me to solve this issue with your plugin.

Thanks,
Jochen