Why so red? IntelliJ seems to think every declaration/method cannot be found/resolved

Question

I just installed and re-installed IntelliJ. Every Java file is coming up RED. I checked the JDK; it is at 1.6.##. The maven clean install build worked just fine

Answer 1

Tried invalidating cache, reimporting the project, switching to built-in Maven, removing the .m2 folder and its parts, - nothing worked. After I had completely broken Maven (while it had built the project successfully before, now it couldn't anymore), I had to reinstall it from scratch. It helped!