I'm not sure whether you ever sorted this out, but I received the identical error when first trying to setup roundcube. It was because I had set the email domain in main.inc.php. The following setting:

// Automatically add this domain to user names for login
// Only for IMAP servers that require full e-mail addresses for login
// Specify an array with 'host' => 'domain' values to support multiple hosts
$rcmail_config['username_domain'] = '';

If this is not left blank, it can cause the precise error you described.

Cheers,
Fiver

The setting you're referring to is blank in my configuration, and my issue is still unresolved.

My bug report was dismissed because I am unable to provide a test account.

but
Error that the service does not automatically creates a directory. Made for example. Where is wrong?