I was getting the same error for a wile and figured it out. I have noticed that when I start my DMS task in the CloudWatch logs I could see that DMS was loading and preparing all the files and databases schemas. Since I have a lot of schemas and tables it took more than 120s to execute and it got timed out and I have received the same error.

*How to fix: Since the problem is at the source go to Endpoint and search for you Source Endpoint and got to Extra connection attributes and enter valued for executeTimeout to 3600.

Ref: https://repost.aws/knowledge-center/dms-error-canceling-statement-timeout