I just tested my TWR-K60F120M board with TWR-SER and it works correctly with both old 31.21 and new 31.26 OSBDM interfaces.
I used hello2 example code.
Are you sure that you tested board with the same configuration as before? Typical issue is when MCU twr board is configured for obtain clock from TWR-SER board and you leave board as standalone after OSBDM firmware update.
Anyway, if firmware 31.26 makes some trouble, you could revert this firmware back to older version via PEFirmwareUpdater utility:
http://www.pemicro.com/downloads/download_file.cfm?download_id=348
In attachment you can find old 31.21 OSBDM firmware for Kinetis boards.
I hope it helps you.
Have a great day,
RadekS
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