Hi again,
I tried the whole troubleshooting part – disabled all plugins and switched to the default theme – but the problem prevailed.
However, I discovered something else (in and outside troubleshooting mode): Once I deleted the original (parent) comment permanently the sub comment appeared again. Still, no notice appeared e.g. ““The original comment of this thread does no longer exist.” What would be the standard WordPress behaviour here? If everything is disabled and the issue (if it’s abnormal behaviour) still exists it should be a wordpress problem I suppose? At least I know now that it is not a theme/plugin related issue.