Specifically the German constitution. Or as also worded in the law "the free democratic basic order of the FRG" -"die freiheitlich demokratische Grundordnung der BRD".

What this phrase means specifically is defined by decisions of the federal constitutional court and includes things like basic human rights, checks and balances, the independence of courts, the multi party system etc.

Disrupting or trying to abolish those basic democratic laws is considered as trying to build a dictatorship or other form of unjust system.

I don't know the specifics about the KPD case but there are German communist parties, for example the DKP. It's just that the KPD is considered undemocratic.

[–] barsoap@lemm.ee 5 points 23 hours ago* (last edited 23 hours ago)

The KPD in particular got banned for wanting to topple the government by force, also, they took their orders from Stalin. Being a communist as such is far from anti-constitutional, the German constitution was specifically written to be compatible, but it's going to have to be democratic market socialism. What you want to do is heavily lean on Article 14(2): "Property entails obligations. Its use shall also serve the public good." You can expropriate means of production by mere preference, without having to show that it's for the public good.