The anti immigration stance is not the problem in itself, it's a symptom of their xenophobic views.
"The party's prevailing ethnic and descent-based understanding of the people is not compatible with the free democratic basic order. It aims to exclude certain population groups from equal participation in society, to subject them to unequal treatment that does not conform to the constitution and thus to assign them a legally devalued status. Specifically, the AfD does not consider German citizens with a history of migration from Muslim countries, for example, to be equal members of the German people as defined by the party in ethnic terms."
"The party's prevailing ethnic and descent-based understanding of the people is not compatible with the free democratic basic order. It aims to exclude certain population groups from equal participation in society, to subject them to unequal treatment that does not conform to the constitution and thus to assign them a legally devalued status. Specifically, the AfD does not consider German citizens with a history of migration from Muslim countries, for example, to be equal members of the German people as defined by the party in ethnic terms."
Translated with deepl from https://www.verfassungsschutz.de/SharedDocs/pressemitteilung...