The engine attempts to generate mipmaps for cubemap faces, but it is not allowed.

[riccardobl]

jmonkeyengine/jme3-core/src/main/java/com/jme3/renderer/opengl/GLRenderer.java

Lines 2019 to 2020 in 24056d6

	int textureType = convertTextureType(tex.getType(), tex.getImage().getMultiSamples(), rb.getFace());
	glfbo.glGenerateMipmapEXT(textureType);

According to the documentation https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glGenerateMipmap.xhtml

target
Specifies the target to which the texture object is bound for glGenerateMipmap. Must be one of GL_TEXTURE_1D, GL_TEXTURE_2D, GL_TEXTURE_3D, GL_TEXTURE_1D_ARRAY, GL_TEXTURE_2D_ARRAY, GL_TEXTURE_CUBE_MAP, or GL_TEXTURE_CUBE_MAP_ARRAY.

the target GL_TEXTURE_CUBE_MAP should be used instead.

[riccardobl]

Fixed with #1409