I tried a few things, including restarting (both the iPhone, and Xcode), but here's a few things that you can do if you're having this problem:
- Go to Keychain Access, find your developer profile, delete them and re download it from Developer Member Centre.
- In Xcode, go to Preferences -> Accounts and remove your Apple ID, and add it back.
- In my case, doing the above will trigger Xcode to create me a new certificate. I just did that and it worked.
