Does this problem only happen on a specific team or all the teams?
If it exists in one team, you could contact team owners to try the workaround to change certain owner’s role to member and add Planner again. If works, then change role back.
If all teams have the same problem, try to access Planner app by this link https://tasks.office.com/. If users has problem to access Planner, confirm with IT admin to check the health status of Planner in Microsoft 365 admin center > Health: Service health.
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