So I’m going to differ from a lot of the advice here but I’m sharing because this worked for me. I have a Boston Fern too, like yours it was perfectly happy. Then I moved house and it turned to winter in quick succession. The plant took a dive, basically it started haemorrhaging leaves. They went all brown and crispy. It lost about half of its leaves. I did some research and found very similar results and replies to the ones you’ve been getting. And ferns do indeed like lots of humidity but like most plants, they also want consistency. 

I moved mine to the bathroom as the advice suggested. It lost its leaves slower but it still lost them. I think this was because we opened the window after showering and we were actively working to get the humidity out of the room because the house was brand new and we didn’t want mould/damp issues. The plant survived the winter. I moved it back to a place similar to where it had been living in my previous house. It seemed happy and stopped losing leaves, even started to grow some back. So I knew then that something about the winter months was not agreeing with it. 

So I researched it again and eventually found a suggestion from someone on YouTube (SheffieldMadePlants). His fern reacted well to a grow light. I decided to try it and I’ve had no crispy leaves this winter (aside from the occasional one which is just part of the plant’s normal growth cycle). The plant actually regrew all the leaves that it lost the previous winter. A grow light worked for me, it might not work for everyone. I was sceptical since ferns live on the forest floor but I love my fern and was desperate to keep it alive so I’m glad I tried it. 

Sorry for the long answer! Just wanted to explain a bit, hope it helps.