Hi Nuno,
Yes, there seems to be a bug when upgrading from version 1.5.3. You have to change file 'opengoo_1.6/public/upgrade/templates/db_migration/1_6_chivito.php' line 139 from:
CHANGE COLUMN `uid` varchar(255) <?php echo $default_collation ?> NOT NULL default '',
to:
MODIFY COLUMN `uid` varchar(255) <?php echo $default_collation ?> NOT NULL default '',
If you have rolled back to version 1.5.3 after the error just do that change and try again. If you are stuck with the broken installation just delete everything from that file (1_6_chivito) upto line 130 (the line before ALTER TABLE `<?php echo $table_prefix ?>mail_contents`) and try again.