I have no idea why the failed conversion is popping up:
1) One of the remaining members gains the ability to convert if Odd dies.
2) A townie that got converted had the ability to grave rob and tried to convert another, but it failed.
But what's important here is that we had a failed conversion notification appear on N3 when there was a successful conversion on N1 and N2 and when Odd was alive. According to Odd's role, there can only be 2 other cult members apart from him. So by N3 he already had his team members and shouldn't have been able to convert, yet we got the failed conversion notification. I think that would be to limit town from knowing how many players could be converted.