In order to fix recently discovered issues, OpenSea got a new smart contract that also requires open positions to be migrated. All openings that won't be migrated, will automatically expire on February 25.
The migration procedure is available under the "Migrate listings" tab on your profile page. There you will be able to migrate every listing individually.
🔥 Important update: Apparently, there was a phishing email that was sent around. Please only use links on the official website.