You have made many API calls successfully in the past. Now that account holders with Admin level access in L2L can also create API Keys, bear in mind this extremely important rule:
Any Username with an associated API Key must be designated as an Admin level account.
Otherwise, generating an API call, even with a legitimate API Key, will not be successful.
If you are presented with error messages that include the word ' Zero ', making sure the Username has Admin level access is your first step in troubleshooting this error.