I am experiencing this problem too w/ ACF 4.3.2 on a 3.4.2 WP site I am adding some enhancements to. The problem seems related to image upload, my repeater fields work fine.
I was able to solve locally 2 ways:
1. Upgrading to 3.8
2. Leaving WP at 3.4.2 but installing the ACF 4.1.8 from Github page.
Not sure if that helps. Hoping I can get the orig. developer to upgrade the live site to 3.8 and that will solve it in my case…
