Health insurance wants $1500 back pay for their mistake – help?

Okay so I just had this discussion with my work’s HR this morning. Apparently, although I enrolled in the low cost, high deductible option, and my employer submitted my insurance like that, my insurance company accidentally opted me into the higher cost option. I’ve had this insurance for a little over a year before the mistake was caught.

I have been billed the lower amount since both I, and my employer, assumed this was correct. But, this mistake was caught, and fixed, however, my insurance company wants $1500 in back pay for their mistake. I have no involvement with the opting in process outside of telling HR what insurance I want.

For context, I have little to no knowledge about how health insurance works (I get confused very easily about things like this), I work for a small business, and my insurance provider is through Blue Care Network.

Can I fight this, or am I screwed over? My employer is willing to work with me with a payment plan, but I don’t feel I should have to pay for a large insurance company’s mistake.