To report an error I must be where the error is. And that was my initial question: How to get to the post where the youtube video was used by mistake, preventing me to use it where it should be.
The error is that a performance was improperly added as a "web cover" here: Smells Like Teen Spirit ..., so one would click the Report Error button as one would under a regular performance.
There are a number of problems with the current SHS policy of favoring the error report over a proper submission in this type of situation, but that's a different discussion. Nevertheless, the biggest problem is that filing a report on a web cover doesn't trigger the creation of a Discussions tab as opposed to other SHS entries exampled here: Wer fragt nach mir .... This means that contributors can't (at least readily) tell if a report has already been filed.
It's of course a personal decision, but due to the lack of a Discussions tab and editors rejecting the resulting duplicate reports, I no longer file them.