Well, the log reports that roundcube database session table is corrupt. I would try to delete the table and to re-create it by ...
-- Table structure for table `session`
CREATE TABLE `session` (
`sess_id` varchar(40) NOT NULL,
`created` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`changed` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`ip` varchar(40) NOT NULL,
`vars` text NOT NULL,
PRIMARY KEY(`sess_id`),
INDEX `changed_index` (`changed`)
) TYPE=INNODB CHARACTER SET utf8 COLLATE utf8_general_ci;
Maybe you have to restart mysql service. Then try again.