Error when trying to give permissions to users and adding them to the site

Hi,

i'm facing an issue with a public website (SharePoint server 2010) where i can't add any users (windows users) or grant permission for them.

I checked the Authentication Provider for the web application and Windows Authentication NTLM is enabled, the IIS also enable windows authentication for that site. 

the issue happens when i go to Site Action --> Site Permission --> Grant Permissions --> I select users after searching for them and once i click OK Exception is appearing. 

I check the logs and all i found is:

10/12/2014 12:16:15.84 	w3wp.exe (0x07C8)                       	0x0AA0	SharePoint Server             	Logging Correlation Data      	9gc5	Verbose 	Thread change; resetting trace level override to 0; resetting correlation to a17a120f-ebf7-4de7-aeb4-91c0ae7be28e d07aff74-632e-4bfe-93d0-415b93f53f6a	a17a120f-ebf7-4de7-aeb4-91c0ae7be28e

Note that this happen all of sudden, i was able to add users / groups, users to groups, grant permissions directly but no i can't do none

My website is grating permissions also for Anonymous users and it's working fine

This is production server site so i can't try lots of options unless i'm sure of them

Event viewer is not capturing any error 

Please help