What the Semperoper is offering is the chance to watch part of a rehearsal a few days before the premiere following an introduction to the work and the production.

The terms “open rehearsal” / "dress rehearsal" come fairly close to this:

“An open rehearsal is a rehearsal that the public can come and watch. In opera houses the dress rehearsal is often an open rehearsal.”
https://simple.wikipedia.org/wiki/Rehearsal

The Royal Opera House in London has a programme of open rehearsals for “Friends” of the opera – these are tickets to watch a dress rehearsal prior to the premiere. I have been to one of them with my school.
Here is the link: https://www.roh.org.uk/tickets-and-events/friends-rehearsals...

The key differences between the ROH and the Semperoper are that the public cannot watch the entire dress rehearsal and that there is an introduction in Dresden. I would therefore add "taster".

I would suggest that “open rehearsal taster” or "dress rehearsal taster" probably come closest to the meaning, and the accompanying text below then explains that it includes an introduction to the work and the production, too.