How to fix 403 in app service with postgres flexible server?

Question

We have an existing app service and are migrating the data and application from mongodb to a flexible postgres-server database.

The stack consists of 3 private endpoints with network interfaces, for the frontend, the keyvault and the postgres-db. Furthermore the stack contains a virtual network.

The app service consists of a frontend in vue/typescript and a backend in Quarkus java 17.0. Authentication is done by Microsoft Entra.

After deployment the application returns a 403 in an incognito browser.

When cookies are already present in a normal browser, so when deploying the application in the previous state with mongodb, followed by deployment of the app service and the postgres db, the application works. After removing the cookies the application also returns 403 Forbidden.

After authentication with for instance Postman, API's in the backend return responses.

Networkaccess for the app-service and the postgres flexible server are set to no access restrictions.

In Application insights the GET-requests to the app service are logged with httpstatuscode 403 Forbidden.

Unfortunately the error only states HttpServletResponseImpl.sendError without further explanation.

Can you please point me to a way to solve this problem? If you need more information please let me know