Hello Youness and Safae,
As far as the translation of the term is concerned, I found several translations. It depends as you know on the context. Here are two of my suggestions:
Millennium Bug: معضلة الحقبة الألفية
Millennium Bug: فيروس الحقبة الألفية
or just as both of you said before:
مشكلة
الألفية أو آفة
الألفيةBest of luck for those who are facing the challenges of translation even in summer
Ikram