We upgraded zabbix server from 2.0.3 to 2.2.4 version.

After the upgrade we got the following error in the frontend:
Database error
The frontend does not match Zabbix database. Current database version (mandatory/optional): 2010033/2010033. Required mandatory version: 2020000. Contact your system administrator.

And the zabbix server logs show the following error:

2593:20140721:120050.159 [Z3005] query failed: [1091] Can't DROP 'value_changed'; check that column/key exists [alter table events drop column value_changed]
2593:20140721:120050.160 database upgrade failed

We are using a mysql (5.5.27) database.

Can you please investigate and let us now how to fix this problem.