I use the following method to get list of contacts from a campaign but it does not work anymore since a few days ago (no changes on my code):
<response uri="/crm/private/xml/Contacts/getRelatedRecords">
<result>
<Contacts>null</Contacts>
</result>
</response>
Could you please check if there is any changes recently on your end that cause this problem.
Please let me know if you need API Token to test.
Thank you.
Nugroho