What happens is that your party gets a quest from Maximus to retrieve the Souls Jars from the warlocks in Mount Nighon and give them to him. If they complete this quest before the Wizard is promoted to Lich, the (empty) Lich Jars needed for that promotion are no longer in the Halls of Mist when your party goes to look for them.
There is a patch which cures the bug, but it only works if the bug has not struck. Once the bug strikes, the only way to cure it is to use an editor and edit in an empty Lich Jar for each party member, as you found out.
Your party can go and get the Soul Jars - that's OK. What they must not do is to give them to Maximus. In practice, it's a lot easier not to go and get them in the first place. It removes any temptation. )