I think it might be in conflict to be using both cordova-plugin-wkwebview-engine and cordova-plugin-wkwebview-file-xhr.
First make sure you are using the latest version of the iOS platform and Xcode. You can change this in Monaca Cloud IDE’s Build Environment Settings menu.
Then remove your WkWebView config lines from config.xml and both wkwebview plugins you have installed. Then choose WkWebView from the App Settings for iOS menu and click Save. This will re-add the correct config lines and the cordova-plugin-wkwebview-engine plugin.
If you are running into the XHR problem, then you can swap out cordova-plugin-wkwebview-engine for cordova-plugin-wkwebview-file-xhr. But you shouldn’t both enabled at the same time. Note that if you select WkWebView and save again from the App Settings for iOS menu then it will re-add cordova-plugin-wkwebview-engine even if you previously removed it.
If that doesn’t work, usually we would expect that the problem is a third-party plugin that hasn’t been updated but since you only have those core plugins, this shouldn’t be a problem.