If deleting a discussion-reply that has-a-like an error occurs, need to unlike first. It is possible that the same problem may be seen if a discussion has a favorite or other ancillary record attached.
If deleting a discussion that has-a-discussion-reply an error occurs, most likely because it would cause a phantom discussion reply. My test case the discussion-reply had-a-like so it is possible the cascade delete worked for the discussion-reply but the like prevented the discussion-reply from being deleted as indicated in first paragraph- sorry I did not have time to research further today.
I have forked a copy of the software (rbianco3) and will try fixing this and updating this as I find out more.