I get a “DUT mismatch error” from CBPro when I plug in the board and start CBPro.  This is assuming you’re using an EVB in the default configuration. You’re not wiring from the field programmer board.  For details about wiring from the field programmer board please review the user guide. It provides detailed information: https://www.silabs.com/documents/public/user-guides/ug286-clockbuilderpro-dongle.pdf

1. Please make sure that the part is not in Reset. Verify that there is no jumper covering the reset (RSTb) pin.  When this pin is held low the part will be in reset and CBPro will not be able to read/write to the part to access register information to determine the correct part number. This would hold the part in reset and you will get this EXACT error.
2. Find the user guide for the specific board and verify all jumper settings. Make sure all SPI pins are connected. The device is not communicating to the chip if you are reading back FF.  Please keep the board in SPI mode.
3. Double check that the driver was programmed into the board correctly by checking the device manager.  Go to device manager and pull up the Silicon Labs Si538x/4x EVB USB driver and click on Events.  Can you push the "View All Events" button. Make sure the board is plugged in. (attached image). If anywhere you see ‘ ~sievb’ this is an indication that the board was not programmed correctly at the factory. Please contact tech support.

4.  Check Power:  USB Power vs External Power:  Can you  verify several of the test points making sure you're powering from USB?  VddA TP1 should be 3.3V. TP2 should be 3.3V, TP23 should be 1.8V. This would verify that the chip is being powered correctly. If you are getting very different numbers this suggests a power problem through the USB path. If there is a problem here could you try powering the board Externally from 5V supply.  The procedure to do this is in the user manual that you've been reading. Again, measure the test points.