'Error during AJAX communication' only for some users

[StefaniaS]

Hello,
I'm using OTRS in different languages and we found that for some agent users, when they are tring to create a new phone or email ticket they get the error 'Error during AJAX communication'.
It happens for two spanish users when they try to select the queue and the service while creating a ticket from agent interface; I've tried to do the same steps that they do because I was thinking to a special character problem, but everything works fine and I didn't get the same error.
It seems they are the only ones that are facing this error, does anyone has a solution for this?

Thanks
S.