How often runs your scheduled event? This can happen for instance when a scheduled event is running and a new one is already starting and also interacts with the same objects as the first still running request.
But there is also something in the release notes of 9.18: https://docs.mendix.com/releasenotes/studio-pro/9.18/
We improved the deadlock avoidance on task queue operations for SQL Server. (Ticket 155244)
So updating to a later version might also fix your problem.