Looks familiar? Old habits die hard it seems.
It used to be the East German SozialEinheitspartei Deutschlands (SED) - Socialist Unity Party Of Germany - the Communist-dominated unity party created out of the merger of German Communists and Social Democrats in the Soviet Zone of Germany after World War II.
Its strongest in the former East Germany though through rebranding, its made some headway in West Germany. The reason the SPD doesn’t want it as a coalition partner is because of its Communist past. Its the one party in Germany no one wants to ally themselves with. Nowadays it called Die Linke - German for The Left.