Hi there holthd,,,
Firstly, you mentioned >>>After activating the device again it works for a short period of time and then it's the same all over.<<< So ,, Since the reactivation fixed this problem for some time and this user is the only one having such a problem . Hence I suggest first of all if it is possible to provide this user with another testing BB for some time and see if the same problem would take place with the testing machine or not ????
Moreover, did you google it ??? and check some similar cases ???
http://www.google.com/search?q=Illegal+device+PIN+in+Black+Berry&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-aBest Wishes