The plugin needs to store the API credentials. You can encrypt these by setting an encryption key in your “wp-config.php”.
First, generate a large, random key. Edit your “wp-config.php” and add
define('ONOFFICE_CREDENTIALS_ENC_KEY', 'b4sx923...23jxlj');. Then you need to go to “onOffice > Settings” and save everything so that your credentials will be encrypted.
Note that if you change this encryption key, you will need to re-enter you credentials.