I just set up SSO(Single Sign-on) in my DEV Sandbox with OKTA service provider. it worked perfectly for a while and after that, it stopped working. The error is "We can not log you in because of an issue with Single Sign-on, Contact your salesforce admin for help". the strange thing here is, I have checked with the OKTA team, in their logs it saying that successfully authenticated each time we try but it showing Error from SFDC login page. 

All configuration was perfect because it worked a while ago. I am testing in DEV Sandbox. any limitation on it. do you have any idea what causing this?