Webmail.domain.com and domain.com/webmail

Hello all forum members.
I found a strange thing.

When I access to my Rouncube with sub-domain "https://webmail.mydomain.com" the site gives a security error.
But when I access it by domain: "https://mydomain.com/webmail" all is ok.

As I remember the SSL worked on the sub-domain in the past.
Same thing is happening to my customers.

Anybody has this problem or has a solution to it?

Thank you.

It is ok.

As SSL certificate is provided only for www subdomain and subdomains you added manually.

This is how AutoSSL works in CWP for now.

Thank you for this clarification