Hi @japel
Note: Please do not create duplicate posts:
http://support.advancedcustomfields.com/forums/topic/loading-field-values-on-front-end-form/
As written in the previous thread:
In the database, the value saves as a:2{i:0;s:5:”value”;i:1;s:12:”anothervalue”;}. This is technically a string, but when WP loads the value, it converts it to an array!
When you update your value, don’t update it with a string, update it with an array.
The array should contain the values such as “value” and “anothervalue”
Let me know if you need any extra help.