I failed to notice that I’d left a colon between the collateral txid and its single digit index number when adding a masternode to my masternode.conf file. I think this caused pivxd to crash during initialisation. However, when I reverted to a known-good masternode.conf the crashing persisted. I’m guessing my wallet’s copy of the blockchain was corrupted during the first failed start with the bad masternode.conf file.
I fixed my masternode.conf but had to restore my .pivx folder from a backup to make everything work again.