Spak freezes when starting conversation

I haveabout 80clientsinstalledsparkacross the enterpriseand the following occurs: when user Awants to talk touser B, theSpark clientfreezes.The samehappens when the userBwants to talk touser A.
Thishappensnothing butthese twousersieuser Adoes not happenwith any othercontact in your listandnotuser Bwithno othercontact in your list.

AlreadyprovedSparkandreinstallingJava, but theproblem persists.

From already, thank you very much.