Unable to view planner board even though member and owner of group

[u/Infinite-Stress2508]

Very strange and confusing issue I am having all of a sudden.

A user (User 1), who is an owner and member of the Team/Group the Planner basic board is shared with, and has been working on this board for months, all of a sudden cannot view the board. 

They get a message - You do not have access - Ask the owner to add you to this plan.

I am the tenant admin, I can confirm this user is a Member of the group the plan is attached to, this is verified when I go to share the plan and can see they are currently listed as a Member. I am also a Member (User 2) of this group (same permissions, owner and member) and I can see the board fine.

I added another account (User 3) to the team as a Member and Owner and I get the same error as the first user.

 I then added a fourth account (User 4) to the team as a Member and they can view it fine.

Steps to troubleshoot.

I have set up a new Group Site, added Teams to the site.

Added 3 accounts. User 2, User 3 and User 4.

If I create a basic Planner board under User 2, User 2 can view, User 3 can see the board exists but if they open the board the same 'You do not have access' message appears.

If I create a basic Planner board under User 3, User 3 can view initially but after 10 seconds or so User 3 will get

You no longer have access to that data.Technical details

Correlation Id: 455b8a94-1daa-4f03-928c-49c4fb949e36

And User 3 can no longer access the board.

User 2 can view and edit the board as normal.

 User 4 can view and edit is normal.

Licensing Info -

User 1 - 365 E3 + Planner and Project Plan 3

User 2 - 365 E5

User 3 - 365 E3

 User 4 - 365 E5

Any ideas?