I've been living in the Netherlands as a student since August, and the health insurance I had up to about a month ago was the European Health Insurance Card. However, at the end of December, I had to swap to a Dutch health insurance company, as obligated by Dutch law if you are about to start working. Some days ago I received a mail by the company where they state that on top of this month's subscription I also have to pay for the period I've been living in the Netherlands (i.e. 5 months amount!), which sounds absurd since I was insured for that time. I called the company and they informed me that "that's how the law works in the Netherlands"... Is it really this way?
Your EHIC wasn't valid -- it's for temporary stays, and can't be used in the country where you're "habitually resident". Since Dutch law doesn't allow you to be uninsured, I'd say you probably are liable to pay for insurance for the period when you were resident in the Netherlands without valid insurance. But I'm certainly not an expert in this area, and if it's a lot of money I'd suggest talking to someone who is, even if you have to pay.