..And found the problem
On line 4 of custom generated file ("/custom/editor_*.js"), change this:
FCKConfig.SkinPath = 'skins/silver/'; // relative to fckeditor/editor/fckeditor.html
to this:
FCKConfig.SkinPath = FCKConfig.BasePath + 'skins/silver/'; // relative to fckeditor/editor/fckeditor.html
Seems that "FCKConfig.BasePath + " is required, don't know why
Please, can you fix this in next releases?