The subject is problem. It's in the passive. We don't know who is addressing (trying to fix) the problem, because the people are are "doing" something are not named. However, the grammatical subject is problem. A problem is being addressed.
A very, very serious problem in Russia is the complete subject.
You are quite correct to add the "is." It's not grammatical (as a full sentence) without it.
Student or Learner