GLES30.glReadPixels(0, 0, (int)mScreenWidth, (int)mScreenHeight, GL10.GL_RGBA, GL10.GL_UNSIGNED_BYTE, bb);
bb is interpret as an offset in your PBO. Thus you're writing out of buffer (On some drivers this code cause crash). You should pass 0 instead of bb. To retrive the data from PBO use glMapBuffer.