Today, i created a new site collection as requested by one of my client.
Everything was working as expected and it was time to give Full control access to client.
Life is full of uncertainty so is SharePoint ;).
As i was in process to give the permissions was got hit by below error.
Troubleshooting1. Tried to give user access on other site collection but same error.
2. Tried to give user access on other web application but found out that issue is occuring for one web application.
3. Check event Logs but no luck.
4. Created a new site collection and web application same problem still occurs.
SolutionPerformed an IISRESET and everything was back to normal.
Happy SharePointing :)