Hi! This could happen when there is a comma in the list of interfaces (in /etc/default/isc-dhcp-server). Please check the output of | debconf-show isc-dhcp-server for commas in the 'isc-dhcp-server/interfaces' variable or the content of /etc/default/isc-dhcp-server and look for commas in INTERFACESv4 or INTERFACESv6. To specify more interfaces a space between the interfaces is sufficient.
best regards, Adi Kriegisch
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