I managed to find something not related to Electrum directly but a fork for it for an altcoin.
The alt's Electrum wasn't made from scratch, it says it was forked from Electrum. Therefore there's a high chance this error comes from a prior version of Electrum, that was later removed. Electrum developers get used to doing these changes.
Your transaction is most probably not signed because this error happens when Electrum can not derive the correct key using the information provided in your wallet. It requires your
change value in your derivation path be either 0 or 1.
OP is probably running an old version that was upgraded from an even older? Can't guess of anything else.
I'd back up my seed phrase (if I hadn't already) and move onto 4.1.5+.