No idea why because I don't know anything about your OS or your version of AE or your project. Try this. Create a new AE project that is 8 bit. Create a new standard HD comp that is 10 seconds long. Add a shape layer or a text layer or a solid to the project. Save your project and then see if you can add it to the AME. If you can, then there's something in your composition that is causing the problem and you need to give us a complete description of what is in the composition. The easiest way to do that is select all of the layers in your timeline and press the U key twice to reveal all changed properties or added effects. Then post a screenshot. If you project contains pre-comps then a flow chart and the timelines from he nested comps will also help.