The cassette cable should not connect to an amplifier. The connection is between DX21 cassette port (DIN) and cassette player/recorder 3.5mm plug (x2) and possibly 2.5mm plug (x1)
I believe the memory protection setting should be OFF - also.
Unless the backup dump is coming from MIDI rather than cassette,the MIDI cables are not involved.
Manual page 3 - section 6
http://www.synthfool.com/docs/Yamaha/DX_Series/Yamaha%20DX-21%20Owners%20Manual.pdf
On Monday, 16 September 2019, 23:54:34 BST, hamsterdrum85@... [YamahaDX] wrote:
? I received a TRS80 cassette cable today and I can’t reset the factory settings. I don’t know what I did wrong. I changed the battery, connected the MIDI in and out, and connected the cassette cable to my amplifier. I keep getting ERR. CASS.
Lee Borrell