What went wrong on Dr. Olsen's return trip to Earth?

The answer is pressurization problems.

Explanation:

In "Mission to Mars" Dr. Olsen tells that when they were coming back to earth they went through pressurization problems. The Soyuz cabin was lacking oxygen, so they had to add more of it. Fortunately, there was not a disaster because the crew was prepared for those types of situations.

On Dr. Olsen's return trip to Earth, there were pressurization problems.