I searched a lot and I couldn't find a similar error somewhere else. I have this error in my eshop whenever I am in the payment page and cancel the procedure. Then I redirect to this error instead of the shop page. After the error, the whole Woocommerce part of the website is not functional and I have to clear Browser history/cookies and run it again.
Catchable fatal error: Object of class __PHP_Incomplete_Class could not be converted to string in /var/www/vhosts/th16529w21.vorschauseite.eu/tourist/wp-includes/kses.php on line 1245
And this is the part of the kses.php that mentioned above:
Edit: I use the plugin wirecard payment gateway for the payment. And the error raised on cancelation of the payment on this plugin.