Recently I posted "A call to BKS holders" urging BKS holders to upgrade and configure votes.
It seems the client (or the network) doesn't like e.g. asset votes in blocks before the protocol was switched to 4.0

If you encounter problems minting blocks with the client version 4.0.1, please remove all asset votes.
If you have registered a data feed from @Cybnate or @cryptog, you can wait until the data feed providers fixed it (one of two already did, although the finding is only a few hours old), or unsubscribe from the data feeds and reset the votes manually.

Short version:
https://discuss.nubits.com/t/mandatory-upgrade-all-minters-need-to-upgrade-to-b-c-exchange-4-0-1/3723/129

For a long version begin reading here:
https://discuss.nubits.com/t/mandatory-upgrade-all-minters-need-to-upgrade-to-b-c-exchange-4-0-1/3723/90