This is makeindex, version 2.15 [MiKTeX 21.6.28].
Scanning input file GradualeMonasterienseParsSecunda-Messe.idx...
!! Input index error (file = GradualeMonasterienseParsSecunda-Messe.idx, line = 1):
   -- Missing arguments -- need two (premature LFD).
.done (48 entries accepted, 1 rejected).
Sorting entries....done (287 comparisons).
Generating output file GradualeMonasterienseParsSecunda-Messe.ind....done (99 lines written, 0 warnings).
Output written in GradualeMonasterienseParsSecunda-Messe.ind.
Transcript written in GradualeMonasterienseParsSecunda-Messe.ilg.