Looks like this is another one of those "place.php" spots that goes into a choice adventure, like untinker and hatter.
To be completely clear on this, to visit the Arena, you do this:
postwarisland.php?place=concert
To choose a concert at the Arena, you submit a URL like this:
postwarisland.php?action=concert&option=2
The other issues were a result of place.php redirecting to choice.php, but, as you can see, neither place.php nor choice.php is involved in casting a concert.
Nothing has changed in either KoL or KoLmafia regarding the "concert" command. If you've been "wondering" why it hasn't been working lately, you'll need to investigate a little more. I'd suggest that you check your preferences that determine whether the war has been finished or not; if it is still underway, we use bigisland.php, not postwarisland.php. That, too, works - but since the character who is using concerts is your "meat tower" (whatever the heck that is), I assume it's in aftercore. If you have trashed your preferences and we use the wrong URL, I'm not sure we'll print an error, even though it fails. As I said before, a DEBUG log would help.