tlsv1 alert unknown ca: SSL alert number 48

[Tobias]

Well there's your problem.
Upgrade Roundcube, or add the code to use imap_conn_options to your 
current version.

http://lists.roundcube.net/pipermail/svn/2014-September/012195.html
   Add config option to specify IMAP connection socket parameters - 
imap_conn_options (#1489948)


On 2016-06-18 15:27, Maurizio Dall'Acqua wrote:
> The version of Roundcube I am using is 0.9.5+dfsg1-4.1