Some users experience an issue when trying to log into Mattermark for iOS with twitter. In most cases this is a result of the core iOS Twitter authentication having expired (can happen when you restore from backup or even just update the OS version)
To resolve this issue, please try the following:
1. Open the 'Settings' app on your iPhone
2. Scroll down to the section containing Twitter and Facebook (right below Game Center). Tap on 'Twitter'
3. Select the username that you'd like to use for Mattermark login
4. Re-authenticate this account (type in your password again and press done in the top right)
5. Last step: Open the Mattermark App again and try to sign in again.
If this does not work, please email us at firstname.lastname@example.org
Our customer success team is here for you.