"We were unable to complete your request because your item or memory limit has been exceeded."

Hi SteFF1024,

The error message you're encountering in Planner, stating "We were unable to complete your request because your item or memory limit has been exceeded," is commonly associated with surpassing the maximum task limit assigned to a user.

Planner imposes a restriction of 3000 tasks per user across all plans. If you've reached this limit, task assignment issues may arise, regardless of whether tasks are open or closed.

To address this problem, try to Delete closed tasks. If you have a substantial number of closed tasks, removing some of these could help resolve the issue.

Here are some useful links:
Microsoft Planner limits - Microsoft Planner | Microsoft Learn
[RESOLVED] Microsoft Planner Storage within a SharePoint Site/365 - Microsoft Community
Cannot assign tasks in Planner - "Your item or storage limit has been - Microsoft Community