one of the German states foundations is responsibility for the Holocaust, which led to the founding of the state of Israel.

There are laws in Germany that make it a crime to condone a crime (forgive, overlook, allow, permit )

Some German courts have ruled that the slogan "between the river and the sea" is condoning the unlawful removal of Israelis or that the slogan is firmly attached to Terrorist Organization Hamas (therefore is by default a criminal statement )

Plenty of people have been fined for chanting the slogan at German protests against the current conduct of Israel in Gaza and West Bank.

There isn't a German law that states "it is illegal to criticize Israel" but laws like the following have been used to punish people criticizing Israel, in Germany:

https://www.gesetze-im-internet.de/stgb/__130.html

Some German courts have thrown out some of these cases, they don't agree the Condone Crime laws can be applied to chanting 'between the river and the sea'