Thanks @strophy
Turns out it's a PEBKAC case. Recovered fine, I must have been mistyping my key.
A question for you my good sir, should I move my keys to a newer format?
Way back in the day I generated my wallet using a generator I found in these forums : https://www.dash.org/forum/threads/how-to-make-a-darkcoin-paper-wallet.116/page-2
This might be the generator I used years ago:
https://github.com/internetape/paperwallet
Back in the darkcoin days I...