There is a conflict between your plugins.
The plugin located in “plugins/custom-contact-forms/” or “Custom Contact Forms” already uses a function called json_api_init() .
Looking at the “Custom Contact Forms” plugin source code on github it looks like it uses some custom version of the WP API. And therefore you probably won’t be able to use these plugins together.
This is in my opinion an issue for the author of Custom Contact Forms plugin to resolve since he uses functions that overlap with the WP API functions.
