It looks like the issue stops at len=6. If I send "ABCDE", it pads to 64 bytes and shows an FCS error. If I send 6 or more bytes, it does not pad. For example, at "ABCDEF" (ie len = 6), the bytes on wire is 60 as expected. After that, it is fine.
I guess it could be a wireshark issue, but again, not sure why it would single out the C6670 EVM board.