This question may be related to a resolved thread.
Updating now from 1.6.8 to 2.4 and got the following Error:
A Database Error Occurred
Error Number: 1091Can’t DROP ‘code’; check that column/key exists
ALTER TABLE `exp_ip2nation_countries` DROP KEY `code`
Filename: /var/clients/client4/web3/web/system/expressionengine/modules/ip_to_nation/upd.ip_to_nation.php
Line Number: 182
i will remove now the ip2nation module. check if that is helping.