In the portfolio class, the API host is set to the sandbox. This is not correct as the users would have to hack your plugin in order for it to work on the production servers.

The best of the best would be offer them a choice to use the production servers on the screen where they enter their API key. I have just done that in the repository: https://github.com/FMCorz/moodle-repository_evernote/commit/dcb3aa1fd61e6be845c33cffdc4de962e2b14e83 Though if you do not have time, at least setting the proper API host is required.