I'm having the same issue as OP. I just logged on from my phone a couple of hours ago, no issue. I've also logged on from my desktop, again, no issue. But suddenly it won't use TouchID, and it won't accept the password. It will give my password hint, and I've typed the password slowly and correctly several times, but it just won't decrypt the vault.
Latest version (6.4.5) on iOS 10.0.2
Following your suggestion to remove and reinstall the app worked, but there definitely seems to be an issue where it randomly locks up and refuses to allow you to open your vaults. There could be times where you don't have access to your account key and can't re-authorize the phone app immediately.
@endemoniada: I hope you don't mind, but I've split you off into a separate discussion since this is account-specific and not related to anything else. For example, that discussion was months old and involved much older OS, 1Password, and device versions.
It seems like you've made changes to your account recently, which would invalidate the credentials. Is that the case? While reinstalling and logging in will work (you can do this using another authorized device, or, at the very least, make a note of your Account Key before deleting 1Password on that one), it's probably simpler to just update your information in the app. I hope this helps!