HTTP 404 is an error code thrown by a web server. It's an indication that the resource requested by the browser cannot be found by the webserver. If you experience this error, first check the extent of what is affected. Is this affecting a specific page or all pages?

Good URLs to test include:

• http://<mt>:<port>/      
• http://<mt>:<port>/arsys
• http://<mt>:<port>/arsys/home
• http://<mt>:<port>/arsys/shared/config/config.jsp
• http://<mt>:<port>/arsys/shared/login.jsp
• Note:  substitute the hostname of the MidTier webserver for <mt> and the port number for <port> 

This knowledge article may contain information that does not apply to version 21.05 or later which runs in a container environment. Please refer to Article Number 000385088 for more information about troubleshooting BMC products in containers.

Use Case 1:

• If all requests fail with HTTP 404, the application is not loaded correctly. Check to see if it's deployed correctly.
  1. If you used the installer, check if arsys.xml file exists in the Tomcat server under Tomcat\conf\Catalina\localhost\arsys.xml.
  2. if so, open it and check if the directory that the docBase parameter is pointing to is correct. It should be the Mid-Tier installation directory.
  3. If everything is in place, check Tomcat's catalina log and localhost log. When Tomcat starts, it will deploy all the applications. If there is a problem, it will record the reason why it can not proceed.
• If there is a specific page that results in HTTP 404, there could be something wrong with the installation or configuration.
  1. Check if the home page server is set up in Mid-Tier's Config Tool.  
  2. If that's correct, check Tomcat's log files again (catalina, stdout and stderr), look for any exceptions that could relate to the specific problem.

Use Case 2:

• HTTP404 Message -The requested resource /arsys is not available is received while login to mid tier.

1. Check the space from database and make sure it is not full.

Additional Checks

• Verify directory having only arsys.xml <tomcat>\conf\Catalina\localhost,
• Verify that there is no other arsys.backup.xml( with extension xml) as they will be looked for deployment.
• If arsys.xml exists, confirm that there is no arsys sub-directory under the Tomcat/webapps directory.

• Sometimes the issue occurs when you update the Java used by the Midtier server.
  Check the version        Command:   java -version

 If the Java version is higher than 255 such as 1.8.0_261, the following issue occur:
Remedy Mid Tier does not start, it gives 404 error.
For more details and for resolution refer below article. 

https://community.bmc.com/s/article/Remedy-Midtier-Midtier-is-not-working-after-upgrading-Java-to-1-7-0u261