I think a key element in problem-solving problems in relationships is constructive criticism and talking about stuff in a neutral or a positive way.

The same is true for the other end. You want to accept criticism without immediately getting defensive on instinct. Actually think about what they told you and try to find ways to improve. And if you are the partner providing the criticism do not try to immediately propose solutions but describe the problem first. Finding the solution is something to work on together once both are on the same page.