There is a bug still. Please fix your Dovecot config manually, checking for missing } - we are currently working on it with Jamie.
Additionally, remove manually all ssl_ca directives from all blocks and make sure that ssl_cert directive is ending with .combined and not .cert. This is failproof method to make it work in the future, even before we patch it eventually for the next Virtualmin 6.12. At the moment it fails because of having ssl_ca in the block.