Symptom
When we are browsing any site hosted on IIS and then we are getting HTTP Error 503 Service Unavailable error.
Cause
Some required IIS components are NOT loading properly. e.g. (The Module DLL <path-to-DLL> failed to load. The data is the error.). This can happen when that feature is not installed properly on the server.
Sample errors for reference.
- The Module DLL C:\WINDOWS\System32\inetsrv\webdav.dll failed to load. The data is the error.
Resolution
First check if the Application Pool for server is started, and if stopped restart it.
If the Application Pool is started and stopped again then review the Windows Application Event logs for errors with source “IIS-W3SVC-WP”
The Module DLL <path-to-DLL> failed to load. The data is the error. Please find below screen shot for reference.
Please check if system is referring failed to load DLL in "C:\Windows\System32\inetsrv\config\applicationHost.config" file. If yes, identify the associated Role/Service and install it on server. In this case "WebDAV Publishing" IIS Module. Please find below screen shot for your reference.
Note: Depending on server configuration, there may be different DLLs causing this problem one by one. Other "failed to load DLL" errors for your reference.
- The Module DLL C:\WINDOWS\System32\inetsrv\iiswsock.dll failed to load. The data is the error.
- Solution: Please install Web Socket Protocol IIS Module
- The Module DLL C:\WINDOWS\System32\inetsrv\warmup.dll failed to load. The data is the error.
- Solution: Please install Application Initialization IIS Module
- The Module DLL C:\WINDOWS\System32\inetsrv\logcust.dll failed to load. The data is the error.
- Solution: Please install Custom Logging IIS Module
- The Module DLL C:\WINDOWS\System32\inetsrv\authcert.dll failed to load. The data is the error.
- Solution: Please install Client Certificate Mapping Authentication IIS Module
- The Module DLL C:\WINDOWS\System32\inetsrv\webdav.dll failed to load. The data is the error.
- Solution: Please install WebDAV Publishing IIS Module