Is it relatively common or is it rare for a judge to determine that a part of a contract is ambiguous?

It is a judge's job to determine whether a contract is enforceable and it is not uncommon for a judge to find that a contractual provision is ambiguous. It is not ambiguous just because one party says so. Even if it is ambiguous does not mean it is unenforceable; the court will attempt to determine the parties intent, and for that parol evidence may be considered. There is a general rule of construction that holds that a provision is interpreted in favor of the party who did not draft the provision, but that is of limited benefit unless you can find evidence of what the intent was. I am not sure where you are in the process, so if this regards a pending divorce, you need to have a divorce lawyer who is familiar with prenups involved.

A provision in an agreement is not ambiguous just because it disfavors one of the parties. Generally, a provision is only ambiguous to the extent its meaning is subject to more than one reasonable interpretation. Additionally, it is common that parol evidence will only be considered if a provision is found to be ambiguous. Application of this general rule will depend on the laws governing the agreement. There is also a general rule that courts will construe any ambiguities against the drafter of the agreement. In this case, your husband. One thing to look out for though is whether there is a provision within the agreement that states that the contract is not to be construed against its drafter or be construed more strictly against either of the parties. A provision like that is likely to nullify the general rule. I would advise you to seek legal counsel to address any nuances to the general rules and to review your specific situation. Good luck!

You have received two good answers to this question. I would just add that you should not be intimidated by threats, especially if the agreement was negotiated with both parties having independent legal counsel. If you have a lawyer who understands premarital agreements you should be able to rely upon their expertise to guide you through. If not, you need to get a family lawyer who does understand premarital agreements and pursue justice.