More mods here than a 1950's high school and OBMM can't export a load order like FOMM can, so I'll just describe the problem.

When I get the paladin to kill me with the Cursed Mace, I get the stage 60 update in the journal twice. That's the "I have goaded Melus Petilius into attacking me with the Cursed Mace. I have been transported back to the Shrine of Molag Bal."

Then when I approach the shrine, Molag Bal says I've failed the quest.

Is this a known problem with any specific mods? I'm running the latest OOO, which alters the gravestone and has a quest related to Melus Petilius, so I'm suspecting that.

Have you completed the Path of Iron quest i.e. taken the claymore from the grave?

And in OBMM you can just click Import/Export and then click view load order. Easy as pie.

No.