openmw-essimporter can't convert OXM_CHALLENGE.ess to .omwsave

• Is the problem OpenMW specific or does it also happen in vanilla Morrowind? It is specific to openmw. Vanilla morrowind is able to load it.
• What is your operating system? Windows 11 [Version 10.0.26100.4061]
• What version of OpenMW are you using? OpenMW-0.49.0-RC8-win64
• What version of Morrowind are you using (i.e. retail CD or Steam)? What addons (Tribunal, Bloodmoon) do you have installed? What language is your Morrowind install? Gog version of morrowind with all official addons and plugins.
• Do you use any mods? If so, does the problem also occur in a clean vanilla install without any mods? I do not use mods.
• What are the exact steps to reproduce the problem? Download save file from https://en.uesp.net/wiki/Morrowind:OXM_Challenge#:~:text=The%20save%20file%20can%20be%20downloaded%20here. Put it in "C:\Users\frolo\Documents\My Games\OpenMW\saves" run openmw-essimporter.exe -m "C:\Users\frolo\Documents\My Games\OpenMW\saves\OXM_CHALLENGE.ess" -o "C:\Users\frolo\Documents\My Games\OpenMW\saves\OXM_CHALLENGE.omwsave"
• What did you expect to happen? What happened instead?

E:\games\OpenMW 0.49.0>openmw-essimporter.exe -m "C:\Users\frolo\Documents\My Games\OpenMW\saves\OXM_CHALLENGE.ess" -o "C:\Users\frolo\Documents\My Games\OpenMW\saves\OXM_CHALLENGE.omwsave"
Loading config file: E:\games\OpenMW 0.49.0\openmw.cfg
Loading config file: C:\Users\frolo\Documents\My Games\OpenMW/openmw.cfg
Logs dir: C:\Users\frolo\Documents\My Games\OpenMW/
User data dir: C:\Users\frolo\Documents\My Games\OpenMW/
Screenshots dir: C:\Users\frolo\Documents\My Games\OpenMW/screenshots
Error: unknown record REGN (0x5614)
Error: unknown record CONT (0x884c)
Error: ESM Error: Expected subrecord INAM but got XNAM
  File: C:\Users\frolo\Documents\My Games\OpenMW\saves\OXM_CHALLENGE.ess
  Record: INFO
  Subrecord: XNAM
  Offset: 0x16391d

E:\games\OpenMW 0.49.0>