The problem is here https://github.com/MrTJP/ProjectRed/blob/master/common/mrtjp/projectred/core/BasicGuiUtils.java#L56

The line references "guibackground.png" but the file is named "GuiBackground.png" and isn't found for loading. Renaming the file in the jar archive to "guibackground.png" fixed the issue or the code can be updated to reference the correct filename.

Fixed via 4d845a6