We found that in some rare cases from users report, when to many oauth tokens are created and exceeds the API limit set by zoho. Which in turn returns Wrong Credential error. This can be fixed by deleting some of the oauth tokens manually in your zoho account here - https://accounts.zoho.com/u/h#setting/authtoken >> MyAccount >> Settings >> Active Auth Tokens.
Also ensure to follow these simple steps properly
1) Got to Settings Page , Enter Username , Password and click "Authentication Key", The "Authentication Key" will be automatically generated.
2) Save Settings.
3) Click on "Fetch CRM Fields" This action will fetch the crm fields from the zoho crm.
4) Now go to "Lead Form Fields" or "Widget Form field" select the fields you want to display, and click "Save Field Settings".
5) And use the short codes according to your needs.