They refuse to answer e-mail or telephone and refused a registered letter
Each owner of the property is a tenant in common (including you). Being a tenant in common entitles one to a proportionate share of the rental income of the property, but it also obligates one to pay the proportionate share of expenses. Obviously, you and the other tenants in common have been harmed by the person who isn't paying up, and since attempts at resolving this dispute without involving lawyers have been unsuccessful, it's time to hire a real estate litigation attorney. The likely remedies available to you here include bringing an action for partition under Article 9 of the Real Property Actions & Proceedings Law (RPAPL), or an action for reimbursement of expenses under RPAPL 1201, and you also may be able to sue for unjust enrichment as well as for an accounting. If you have a written agreement that governs your relations with the other tenants in common, then that document is obviously critical to determining your rights here.
An added difficulty is that you are not the only tenant in common who is aggrieved, and I'm not sure whether you own the property individually or via ownership in shares of a corporation. Given the complexities and value of the real property in question, I urge you to consult an attorney here promptly. In addition, if you own the property through some sort of corporate entity and if you are also a corporate officer or director or manager, you may have a fiduciary duty to act, especially if you have a more active role in managing the company than some of the other tenants in common. Again, the best advice I can give you here is to consult a real estate litigation attorney. It may be possible to avoid litigation here, but you need to know your rights as well as your obligations.