@TeilerThule What you've described is exactly what happens when a pack is blocked or deactivated in someone's account. This usually happens because the product key isn't legitimate, always a risk when buying from unauthorized third-party sellers. However, EA doesn't always block affected packs right away; sometimes it can take months, or perhaps even years, before the keys are identified on players' accounts.
If you haven't already, you can try clearing Origin's cache, and if taht doesn't help, uninstalling Origin "the hard way" and reinstalling.
https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/
https://help.ea.com/en-us/help/origin/origin/manually-uninstalling-origin/
If these steps don't help though, you'll need to contact EA customer support and specifically ask about the status of the product key tied to Pets. If it is in fact not a valid key, you'll need to have it removed so you can buy another copy of Pets, that is if you want one.
https://help.ea.com/en/contact-us/
In the future, it would be best to buy your packs either directly from Origin or from an authorized third-party vendor. Here's the list:
https://help.ea.com/en-us/help/faq/where-to-buy-ea-games-v2/
