Billing Address does update when you check "use my shipping address" box

I have a problem with a store I host. Its running 2.1.1 and for some reason now after the recent upgrade when new users go to check out and it gets to step 2 {address}

If you just check use my shipping address it grays the fields out and when you hit continue it just likes refreshes the page. If you uncheck the use my shipping address and manually fill in the billing address fields it works fine and continues to the payment and shipping section

Anyone got an idea? or an Idea where i can start

I fixed it by switching to another theme… then switching back to our own modified theme… everything is working now


What is the exact file to update please?

unfortunately i am not sure what part of the theme needed updated… thats why i just switched to another them to see if it worked and it did…then i switched it back and it must have copied something it needed… the checkout actually looks different now too.