Error encountered attempting to browse to Jet Web Portal Site.
HTTP Error 503. The service is unavailable
Open Internet Information Services (IIS) Manager
Select the Application Pools
In the Application Pools frame, click on the Jet Web Portal
**Note that the Status shows Stopped**
Right-click on it and select Advanced Settings…
Set Enable 32-bit Applications to False
Click OK button
Right-click on the Jet Web Portal and select Start
Allow for a few seconds, then try refreshing the Jet Web Portal site once again from your browser.
Anyone with whom the access has been granted should be able to browse to the site from within a supported web browser.
Recently, with the release of Windows 10 Version 1709 (Fall Creators Update) or Windows Server 2016 Version 1709, after the upgrade, some web applications do not start, and they return the above HTTP 503 error message. (Article ID: 4050891 - Last Review: Oct 17, 2017)
More details can be found in this Microsoft Support article.