This is related to two old posts:
/topic/1522-an-error-has-occurred-on-uploading/
I stumble to the same problem lately when I tried creating the upload feature. Somewhere, web-johnny provided a sort of debugging script to be added in Grocery_CRUD.php library to check what level of difficulties the error is. When I tried it, I got "The problem is probably simple" and notice there is something wrong with the $state_info->field_name, it's blank and thinking possibly the reason why I got this error "An error has occurred on uploading.". So what I did was I assigned my field name like this: $state_info->field_name='upload_file' and fortunately it works. I'm not sure if there something wrong with my code in my controller that causes $state_info to blank or my "fixed" causes adverse effect later on. My uploading process works fine now.
I'm logging this question perhaps other might have the same problem as mine and also could shed more light with regards to this issue.