@Puddingpapst I'm assuming you were installing the PWA on web right? (not through mobile). I just tried replicating by installing the PWA through Chrome browser, then opened YouTube, Gmail but wasn't able to replicate the issue... didn't get any invalid certificate messages.
For now, maybe you can just go through the actual address on Chrome?
@Ivan Yeah, it happened about an hour after installing the PWA. I mentioned this in the subreddit, too. Moderator replied they are known of this issue, and working on that. I opened this thread to give better and more information about how the problem ocurred for me. As I already mentioned after removing the PWA and clearing the cache afterwards - again - the error was gone. When I reinstall the PWA it's back until I remove and clear the cache again.
That's good to know @Puddingpapst and thanks for sharing the info. I wasn't able to replicate it, but hopefully the issue can be resolved soon.