Renaming of observed seismograms:
SEIS_FILE_z.su -> SEIS_FILE_vz.su
SEIS_FILE_x.su -> SEIS_FILE_vx.su
SEIS_FILE_y.su -> SEIS_FILE_vy.su

Either rename your observed data, or (if synthetic) rerun forward modeling, so IFOS will create observed data, which is correctly named.