As Remy Beijing points out, the grammatically correct form is actually 'their'. He is absolutely right when he says that the preposition 'about' should be followed by a noun or gerund. The gerund here is 'causing', so it needs to have a possessive 'their' before it. However, this form is rarely used, other than in formal contexts.
Of the two options you've given, the first is wrong. The second - 'them' - would be generally seen as correct, as this is what almost all native speakers would say.
This is a classic example of a foreign learner who knows English grammar better than most native speakers do!