Error during "Build Gradle Project"

I am still struggling with my Android build and now I get an error during the "Build Gradle Project". I have been trying to google, testing etc. but have not been able to understand how to solve this annoying problem.

Here is the error I get:

* What went wrong:

Execution failed for task ':unityLibrary:compileReleaseJavaWithJavac'.

> Compilation failed; see the compiler error output for details.


* Try:

Run with --stacktrace option to get the stack trace. Run with --info or --debug...

Error during "Build Gradle Project"