If two account display a streak of suspicious looking games against each other (like a row of schneiders where one always wins and one always loses), the app automatically blocks them from further playing against each other.

This is a measure to prevent players from artificially raising their Elo score by playing against their own fake accounts, but sometimes it also affects innocent players.

Should this happen to you and your regular playing partner, please just both play 1-2 games against other opponents to make the lock go away.