You may have missed the announcement earlier in April (Released: April 2025 Exchange Server Hotfix Updates | Microsoft Community Hub) but Exchange Hybrid configuration is transitioning to a dedicated Exchange Hybrid app in Entra instead of using Exchange Web Services (EWS).
It is important to start transitioning now as starting August 19th 2025, Microsoft is going to temporarily blocked EWS to remind customers to move the the dedicated application.
During these periods, Exchange Hybrid functionalities involving use of EWS (such as free/busy) will stop working.
The planning is as follow:
| Block | Block starting | Block length (days) |
| 1st | August 19 | 2 |
| 2nd | September 16 | 3 |
| 3rd | October 7 | 3 |
| Final | After October 31 | Permanent |
Take action following this article Exchange Server Security Changes for Hybrid Deployments | Microsoft Community Hub or Dedicated Hybrid App: temporary enforcements, new HCW and possible hybrid functionality disruptions | Microsoft Community Hub