What are common problems of couples in their 40s and 50s?

The usual things that couples in their 40s and 50s are facing are:

Midlife crisis:  Many people experience a midlife crisis in their 40s, which can lead to feelings of uncertainty, anxiety, and depression. This can put a strain on relationships.

Empty nest syndrome: When children leave home, couples may experience a sense of loss and emptiness. This can lead to conflict and resentment.

Career changes: Many people make career changes in their 40s, which can add stress to relationships.

Health problems:  Health problems are more common in middle age, which can impact relationships.

People have a tendency to blame their partner or their relationship for their unhappiness rather than take responsibility for themselves. This stage of the marriage is what is sometimes called "the do your own thing stage of the marriage" because the individual needs to take responsibility for their own unhappiness and not expect other people to make them happy. They need to realize that they are not a victim but an agent in creating a life of satisfaction and fulfillment. 

Finding a way to do one's own thing and yet still stay connected takes patience, persistence, negotiation, assertiveness, and compassion which are qualities not always easy to develop without coaching and support.

Being able to give one's partner the space to do their own thing without jealousy and withdrawal takes a high degree of security, curiosity, and self sufficiency. It is what the people who apply Bowen theory call the "differentiation of self", DOS.

Finding an experienced mature therapist who can help the couple navigate this transformation in their relationship can be a challenge. The best way to find such a person is to ask friends who have seen counselors themselves.