RE: Cannot Send Email from Leads using Email Template
1. Check if there is any plug-ins or workflows triggering on create of email.
2. Try to send email directly from the same lead without using email template. This is to confirm if the problem is with email template only.
3. Try to access below two URLs and check which one is succeeded (replace your org address). If the account form opens, need to check how this account is related to the lead. If lead form opens, need to check why system treating leadid as accountid.
https://<YourOrgAddress>/main.aspx?pagetype=entityrecord&etn=account&id=c334f316-5cc7-eb11-bacc-000d3aa2b40f
https://<YourOrgAddress>/main.aspx?pagetype=entityrecord&etn=lead&id=c334f316-5cc7-eb11-bacc-000d3aa2b40f
4. Please share screenshot of your lead email template.