Error "Unable to connect to the server" when trying to migrate IMAP email-account to Zoho

Hello,

When trying to migrate an IMAP email-account to Zoho I keep getting the error-message "Unable to connect to the server" when adding an account to a migration.
I learned from earlier posts that this might indicate an issue with the SSL-certificate on the IMAP server that you are migrating from. Since I am migrating from my own mail-server I made sure the SSL-certificate is valid and from a known certification authority, but still getting the same error.

Could somebody from Zoho have a look what is causing the issue?

Thanks in advance, Ton