I had the exact same problem after upgrading from source 1.15 to 1.16 and not having synced the wallet for a few weeks. Like almost all other Mint wallet updates, I deleted the files in the data directory (except for wallet.dat) and am downloading the blockchain again, which should solve the issue.

The Mint wallet is the only one I have to do that with, the OP should contain a link or torrent to a bootstrap.dat synced with the current wallet to save everyone from having to sync the blockchain manually over several days.