click on acknowledge link in dashboard from master node :

reset() [<a href='function.reset'>function.reset</a>]: Passed variable is not an array or object[/var/www/zabbix/acknow.php:79]
Undefined index: acknowledged[/var/www/zabbix/acknow.php:80]

attempt to acknowledge it :

reset() [<a href='function.reset'>function.reset</a>]: Passed variable is not an array or object[/var/www/zabbix/acknow.php:79]
Undefined index: acknowledged[/var/www/zabbix/acknow.php:80]
You have not enough rights for operation

supposedly this was working fine before :

http://www.zabbix.com/forum/showthread.php?t=16107