I think the situation is like this now:

1) MorphOS originally had the mounting script in MOSSYS:Ambient/scripts/mountisoimage
2) If you (or Chrysalis pack) had saved any modifications to the x-iso-9660 mimetype, mountisoimage was left as a user setting which overrides the default settings and kept it pointing to the original script location.
3) At some point the default mounting script in MorphOS was replaced by MOSSYS:S/MountISO and mountisoimage was removed.
4) Your custom x-iso-9660 mimetype kept pointing to the removed file.
5) If you want to revert the whole mimetype to the official defaults to fix the issue, remove the SYS:Prefs/Ambient/filetypes/application/x-iso-9660 file. And of course you could also edit the current mimetype to execute the MountISO line told in the previous post if you want to keep other changes in it.

This has been a small overlook by the team at that point and should probably have been handled in some other way...


[ Edited by jPV 11.03.2022 - 08:19 ]