The best open source alternative to Microsoft Planner is Taiga.io. If that doesn't suit you, our users have ranked more than 50 alternatives to Microsoft Planner and eight of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Microsoft Planner are Kanboard (Free, Open Source), Focalboard (Free, Open Source), Leantime (Freemium, Open Source) and Planka (Free, Open Source).
Kanboard is a simple visual task board software. Kanboard is a free and open source Kanban project management software. There is no fancy user interface, Kanboard focus on simplicity and minimalism. The number of features is voluntary limited.
Leantime is a project management system for small teams and startups. Designed to help you manage your projects from ideation to delivery. Leantime's Idea Manager allows you to store ideas, communicate them with team members, discuss and move them from idea to validation.
Once enabled, a new Tasks menu will appear in your Nextcloud apps menu. From there you can add and delete tasks, edit their title, description, start and due dates, reminder times, mark them as important, and add comments on them. Tasks can be shared between users.