User is taken from AD, the account is unlocked and synchronization is up to date.
When creating a ticket, the field Reporter recognizes the user but returns a "tje reporter specified is not a user".
This happens with some users, the rest works fine.
I couldn´t get a pattern with the users that failed, and also happened that a user that used to fail, started to work "magically". 
I don´t know what can be happening, can you help me?
Thank you!!