Hi,
Assuming you are not running locally or in a sandbox.
Check the custom runtime settings
https://docs.mendix.com/refguide/tricky-custom-runtime-settings/
These settings affect session timeouts:
If your app is running outside of MxCloud and if it is not just the sessions but a restart of the environment, you might not have set your subscription key in the application settings.
https://docs.mendix.com/developerportal/deploy/licensing-apps-outside-mxcloud/