I get the same error when updating my newly arrive DT50-212 with a cakewalk UM-1G MIDI interface.
This is resolved by the reverting the connection.
From the instruction, it suggests.
MIDI Cable In -> DT MIDI Out
MIDI Cable Out -> DT MIDI In
It generates the error.
When I revert it,
MIDI Cable Out -> DT MIDI Out
MIDI Cable In -> DT MIDI In
The upgrade finishes smoothly.
And then I find someone saying the I/O connection varies on vendors....