Hello, I have a similar issue, and think I know why it is doing this even with custom success page set. But first off, thank you for such a wonderful plugin, exactly what I have been looking for.
I am able to get everything to work, uploading, submit, custom success page. However, if the image is over a certain resolution, instead of the success page, it goes to: http://rnbbrands.com/wordpress/wp-admin/admin-ajax.php
The image is still available for approval and can still be used from media library, it's just that the success page is not shown. Both images I tried are JPG, one is 84kb (408x201) that does show the success page and the other is a little over 1MB (2848x2134) that does not show the success page...
Noticed that the DPI on the larger image was 480DPI, took it down to 72DPI and same issue.
Resized the image to 800x599 and now the success page works.
So, unsure why the resolution would affect whether the success page works or not, but there ya go 8-)
Also, and this is not a problem but thought you might want to know, none of the images can be deleted permanently directly from the UGC menu, have to go to edit the image and then delete.
Once again, thank you very much for the plugin, it is very awesome.
I will update if I find out why the error is occurring.