Claustrophobia
The three soviet astronauts with on board with three more newcomers for 25 days. This was the longest amount of time that six people lived in the cramped station.
Source: americaspace.com
After 25 days, two of the original cosmonauts and Chrétien returned to earth. However, Krikalev, Volkov, and Valeri Polyakov stayed aboard. At least they had more room now.