The package privacyidea-radius is missing the file dictionary.netknights, that specifies the RADIUS attribute privacyIDEA-Serial.
# -*- text -*-
#
# privacyIDEA dictionary
#
VENDOR NetKnights 44929
# Backwards compatibility.
BEGIN-VENDOR NetKnights
ATTRIBUTE privacyIDEA-Serial 1 string
END-VENDOR NetKnights