It's apparently multiple Strikes:
https://www.bungie.net/en/Forums/Post/264988560?sort=0&page=0
Difference being, only one of those Strikes gates off literally the [i]entire rest of the game[/i].
_______
Edit:
Just for the sake of thoroughness, and to vent some of my frustration with this...
https://www.bungie.net/en/Forums/Post/264989665?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264988796?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264982879?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264986531?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264982047?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264987967?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264987556?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264987784?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264986446?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264986112?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264985979?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264985802?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264985605?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264985020?sort=0&page=0
https://www.bungie.net/en/Forums/Post/264984071?sort=0&page=0
The scattershot and, frankly, boilerplate nature of the responses to these reports make me really nervous that this wasn't even being properly escalated up the chain until way too late to do anything about in time ("clear the cache," "show us a video on YouTube of it happening," "we're looking into it but won't say anything more," "check the News page [which has zero acknowledgement of the problem even existing] for updates...")