There are a lot of IIS errors out there.
Way too many, right?

Quick fix

The 500 Internal Server Error is a common HTTP error code indicating that something has gone wrong on the web server. As each website can use an individual error code, the error message may also look similar to one the following messages:

• “500 Internal Server Error”
• “HTTP 500 Internal Server Error” or “HTTP 500 Internal Error” or “HTTP Error 500”
• “Temporary Error (500)” or “500 Error”

These error messages usually occur when the server is unable to specify the problem. The most common causes are incorrect permissions on your files or folders, a PHP timeout, or coding errors in .htaccess. To solve the problem, check the configuration of your site or look for further information in the error log via IIS manager.

Best Solution:

https://www.lifewire.com/500-internal-server-error-explained-2622938

Quick fix

The error 500.19 is an internal server error often occurring on a server using Microsoft IIS software. It indicates that the configuration data for the page is invalid.

To solve the issue, delete the malformed XML element from the Web.config file or from the ApplicationHost.config file.

Best Solution:

https://support.microsoft.com/en-gb/help/942055/http-error-500-19-error-when-you-open-an-iis-7-0-webpage

Quick fix

The IIS error 503 is a server error may occur when you try to connect to a web application. “Error 503 - The service is unavailable” usually occurs if the application pool associated with the web application you are trying to connect to fails to start. This may have been caused by an overload or ongoing maintenance.

To solve the problem quickly, follow these steps:

1. View the system log in the Event Viewer to find the error. If the application pool fails to start, this error is logged in the system log.
2. If you cannot find the relevant event in the system log, continue your search in the HTTPERR log file located in the following system folder: c:\windows\system32\logfiles. Search for “503” to find out why the application pool failed to start.

Best Solution:

https://windowsreport.com/http-error-503-service-unavailable/

https://support.microsoft.com/en-us/help/2619402/error-503-service-unavailable-when-you-browse-windows-sbs-websites

Quick fix

The error 403 Forbidden usually occurs when you are trying to access a directory or page that you do not have permission to access. You IIS web server may provide more specific details on the cause by adding a suffix number after the 403, for example “403.14 Forbidden” (Directory listing denied).

The solution depends on what caused the error to occur. If you cannot find details on the cause, try the most common quick fix solutions.

Best Solution: 

https://www.lifewire.com/403-forbidden-error-explained-2617989

Quick fix

The 404 error message occurs when the web server is unable to retrieve the requested page. The most common causes of this IIS error are that the requested file has been renamed, deleted or moved to another location.

In order to solve the issue, make sure that the file you requested exists on the ISS. Also check if it is in the right location and has the correct name. To find out where the requested file is located, you can use the IIS MMC snap-in.

Best Solution:

https://support.microsoft.com/en-gb/help/248033/how-system-administrators-can-troubleshoot-an-http-404-file-not-found

Quick fix

If you receive error 401 by your IIS, you are facing authentication problems. The error occurs when the authentication to the admin or helpdesk portal fails due to a misconfiguration of authentication settings.

To fix the problem, make sure that your account has the required permissions by verifying the authentication settings in IIS Manager.

Best Solution:

https://kb.netwrix.com/1162

Quick fix

IIS error 400 occurs when the server was unable to process the request sent to the website server. The most common cause of the Bad Request error 400 is an invalid URL, but it may occur due to a different cause as well.

To solve IIS error 400, make sure that the URL you have entered is correct. Typing errors or non-allowed characters in the URL are the most common cause of the Bad Request error. If the error still occurs after verifying the URL, clear your browser’s cache, DNS cache and cookies and try again.

Best Solution:

https://www.lifewire.com/how-to-fix-a-400-bad-request-error-2617988

Quick fix

Warning 9009 is an IIS error of the Application Host. It occurs when the Application Host failed to delete the history directory. The Application Host Helper Service detects any changes in the ApplicationHost.config file and creates a backup in a subdirectory. However, the maximum number of subdirectories is 10. If this number is exceeded, the oldest one is usually deleted. If this process fails, error 9009 occurs.

To solve this problem, stop and restart the Application Host Helper Service (AppHostSyc). The AppHostSyc will then start the service and delete the subdirectory automatically. If it still fails to delete the history directory, try restarting the service manually.

Best Solution:

http://intelligentsystemsmonitoring.com/knowledgebase/internet-information-services/event-id-iis-application-host-history-configuration-185366/

Quick fix

ISS error 301 is a status code that informs the client that the location of a requested resource has permanently changed. If this server-side status comes up unexpectedly, you may want to diagnose the problem.

The first step to solve the problem is to check if there are any incorrect redirect instructions on the server configuration files or your web server software. You can also check the application logs to look for further information of the possible cause.

Best Solution:

https://airbrake.io/blog/http-errors/301-moved-permanently