Importing existing resources change set fails with Index: 1, Size: 1

What does your template look like? What are you importing? Are you importing resources creating a new Stack or updating an existing one? If the latter, are you avoiding any template changes not strictly related to the import, as this will make the import fail?

One thing I've learnt from importing resources is to start small. Start with a template that defines just one or two resources to import and get that working, then modify your template to progressively add more with Stack updates. Otherwise all the retries are very tedious!