Newek

Seems like you're using the Forge MDK 31.1.0. I had the exact same issue with it, even if I made sure to correclty setup my paths and workspace (JAVA_HOME, JRE_HOME and Path). I didn't even have Java 9 anywhere on my PC. I just went for the latest Forge MDK update (31.1.41) and it worked just fine after reinitializing my workspace (deleting the .metadata folder in your workspace emplacement, or directly from Eclipse). Hope that can help :)