Cause of Web apps HTTP Error 503 and WAS event 5189
This error can occur because the Windows Activation Service (WAS) creates a temporary configuration file for each IIS application pool in the folder below during typical operation.
During an initial upgrade phase, Windows Update scans the existing folders and files (outside the Windows folder) and records their paths to be restored after the upgrade. However, because the configuration files are temporary, they are deleted when WAS is stopped.
In the next phase of Windows Update, these previously scanned files and folders are copied to a temporary upgrade location. After Windows is upgraded, Windows Update creates a symbolic link to each folder that was copied to a temporary upgrade location before it tries to restore these files and folders to their original location.
However, because these temporary configuration files no longer exist, Windows Update does not remove the symbolic links.
When WAS tries to start as an IIS worker process, it does not create a temporary folder to write the configuration because of the symbolic links. Therefore, Http.Sys returns an HTTP 503 error.
How to resolve Web apps HTTP Error 503 and WAS event 5189
To resolve this problem, Microsoft recommends to manually delete the symbolic links (Symbolic links can be deleted the same as regular files) that are created by Windows Update. To do this, follow these steps.
Open Command Prompt in elevated mode, copy and paste the command below and hit Enter:
net stop WAS /y
rmdir /s /q C:\inetpub\temp\appPools
net start W3SVC
Hope this post helps!