PLB Arbiter does not assert PLB_MBusy (mbusy) signal properly. It should be valid for the entire period of PLB_Merr.
This problem has been fixed and published for plb_v34_v1_02_a. in the latest EDK 8.1i Service Pack, available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is EDK 8.1i Service Pack 2.