Thank you for your help, but I can't do more too. This webpage isn't mine and the underlying javascript code is very confused (protected, obfuscated?) so I just know that the webview works otherwise as the Chrome in the same device.
I read a lot and did a lot of other websettings, but no result. In the ultimatewebview there is no error in ReceivedError or ReceivedHttpError, only the webpage goes blank after pressing some buttons and in the normal webview shows this error (in the log):
TypeError: y.map(...).flat is not a function
Uncaught TypeError: y.map(...).flat is not a function