I'd say that strictly speaking they are a bit different.
With (a), the focus is on "we have to ... now": there are many problems, and *now* someone or something is pushing/forcing you to solve them (e.g. your boss). The problems themselves may have existed for a while.
With (b), the focus is on "we have ... now": the problems just popped up. However, you're only saying that they are there and need to be solved someday; you're not necessarily going to do it right now.