Thanks for the help, all.
Go to Windows>Console and type the following, skip the first one if your wallet is unencrypted, replacing the password accordingly:
walletpassphrase PASSWORD 500
And
Try again if it works. If not, what does the console say?
Console says null.
NeuroticFish's link looks like it will fix the issue. I recently encrypted an old wallet from 2011. I did not upgrade it to HD, as far as I know. Wish I could return to the old unencrypted wallet...
Edit: I downgraded Bitcoin core to v17.1 and send works again.
However, my transaction is going unconfirmed. It looks like the client is mining a pre-fork chain. "Warning: Unknown block versions being mined! It's possible unknown rules are in effect"