Error on 'Add Email' of Zoho CRM Email Plug-in, where are logs?

Error on 'Add Email' of Zoho CRM Email Plug-in, where are logs?

I get an non-verbose error on a sent email, see attachment.

The only to recipient doesn't exist in CRM, by the settings of  Zoho CRM Email Plug-in it should create a Lead.

Where would I find logs on my desktop, to maybe understand the reason for it.

Thank you for support. CH