Hello @wak120191 ,

You receive a 503 error (Service not available) when the Application Pool the website runs under failed to start successfully. You will see warnings when the application pool is stopped and started and when the first request for a page is made the application pool will stop and an error logged in the event viewer.

This occurs for a number of reasons:
1. The account used to start the application pool does not have the rights to 'Start as a Service'
2. The account is not part of the IIS_WPG local group.
3. The password for the account was typed in wrong when setting the application pool identity for the application pool in IIS.
Once all of these were confirmed do an IIS reset. Should do the trick

Also look at:-

“Service Unavailable” error when browsing to a site hosted in IIS 6.0:

https://help.k2.com/kb000095

There is a 503 error when opening Workspace:

https://community.k2.com/t5/K2-blackpearl-Articles/There-is-a-503-error-when-opening-Workspace/ta-p/77551?nobounce

Kudos ?

Thanks, @KagisoMasha for giving me clue for my case username got changed after enabling active directory. Successfully fixed by doing steps shown in the image.