Servers should agree on valid ballots
As of now, servers do not communicate before they are ready to share their tally. If any honest server have an incorrect set of ballots they will share a tally which is inconsistent with the other honest servers. This makes the final result wrong, because inconsistent shares cannot be combined to a meaningful secret.
To solve this, we can use byzantine agreement to make the servers agree on the largest subset of ballots that at least half of the servers have. This would result in the honest servers tallying consistent ballots, which in the end would produce a correct final result.