That was an annoying configuration where my project was using JDK 11 but I did not see that my Gradle settings were using JDK 16 and that kept giving me the error:
Caused by: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
After I removed JDK 16 from my project, it pointed me to the fact that Gradle had no JDK configured. Hint.
Now that everything is using JDK 11, life is beautiful again in my IDE.