I’ve discovered that this commit Keep the scene and processor lists as SafeArrayList · jMonkeyEngine/jmonkeyengine@6625e57 · GitHub breaks the compatibility with the tonegodgui library.
@t0neg0d, I summon thee!
This error would just require a recompile of ToneGodGui… And btw its a library for 3.0 stable.
You’re right. However, I had to also rename all occurrences of interpolate() to interpolateLocal() inside tonegodgui (+emitter).
The gui works, my game runs but all the 3d is replaced with a solid color… again, can’t complain, it’s a nightly 
3.1 probably breaks a lot of things. Libraries will have to be ported to it. (Even Lemur has to at minimum recompile against 3.1 and when 3.1 goes alpha I will have two branches for Lemur to support 3.0 and 3.1 until 3.1 goes final.)