Planner won't load

Hello everybody,

i'm totalyy new here and found some threads concerning the issue i'm dealing with right now.

My Company is Pretty new in using Microsoft Teams. Only 10 People are assigned to a Teams-License (Free-Version) in Office365. 

Everybody uses the the latest update of the Desktop Client (1.2.00.19260 (x64), August First). I added 3 new Members recently to an existing Team wich uses Planner. 

The issue i'm having is: 

When the new Members want to open the Planner-Tab in the Desktop-Client OR the Web-Planner is just says "Loading", and it says "Loading" for Hours. Nothing happens. Just "Loading" No error Messages at all. 

When the "older" members in the Team (including myself) open the Planner Tab it works just fine within a millisecond.

I already checked the Azure AD Configuration but there is no difference in the user Settings regarding "Old" vs. "New" members. 

Do you know anything About this issue? Is there a Workaround? I've googled and googled but found absolutely Nothing About this. 

Reminder: It doen't loed in Desktop-Client or Web-App. Neither via Teams or directly via Planner-Web-App.

Thanks in Advance

Bjuu82