Another problem. Updated the Kotlin plugin to 1.8.20, now all my build.gradle.kts are broken, it fails with:

Can't find built-in class kotlin.Cloneable

I get the error around every 2 seconds. Is there a workaround next do downgrade the plugin?

Looks like https://youtrack.jetbrains.com/issue/KTIJ-16960/AE-Cant-find-built-in-class-kotlin.Cloneable-on-loading-script-configuration. Make sure Project SDK is correct and reimport the project / delete .idea folder and reimport.

deleting the .idea folder helped, thanks 👍