I assume you applied using the 3-year marriage rule. In that case, they mistakenly applied the 5-year rule to you. Using 3 years would put the registration incident outside the statutory period. That provides one good basis for the appeal.
In addition, determine whether the registration form had any wording saying you must be a US citizen. If it did not, you can argue that you did not claim to be a US citizen when you registered. Voting in local elections is allowed for permanent residents in some jurisdictions, so the act of registering to vote is not always automatically claiming to be a US citizen. In particular, if you can get evidence to show that your particular jurisdiction allows permanent residents to vote in local elections, that helps your case even more.
And you better get a lawyer, because appeals are usually a waste of time without one.
PD: Jan 2003 (EB3 rest of world)
I-485 filed: June 2005 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.