N=2 and N=3 Heisenberg chains are to simple for spinpack, because spinpack uses symmetries and so matrix size becomes to one. Use minimum N=5 sites or switch off all symmetries to see the full base:

config.h:
 #define CONFIG_NOS1SYM 1
 #define Sud 0
 
daten.i:
 verbose=33
 x3 99
 sym_k= -9999
 nud=1,1   # ud-subspace
 a0
 nud=0,2   # dd-subspace
 a0
 nud=2,0   # uu-subspace (same as dd-subspace)
 a0