K2 Cannot Find User When Adding Into Roles

Good day.

I need to add user into a role using K2 Management. However, when I try to search any user, nothing appears.  

Sample:

But when I tryto search the user in the Active Directory folder, the users can be found.

Please note that I was able to successfully add users into roles before but suddenly, users disappeared. 

Kindly help to advise if there's service I need to run or configuration I need to check.  

Thank you in advance.