when looking at zabbix server log with debuglevel 4, several error cases can be observed :

32414:20090122:124032 EINF: 0 ipmi_lan.c(got_rmcpp_open_session_rsp): BMC returned an auth algorithm that wasn't supported: 1
32414:20090122:124032 Cannot connect to IPMI host. Error 0x16 Invalid argument

32414:20090122:124132 EINF: 0 ipmi_lan.c(auth_cap_done): Requested authentication not supported
32414:20090122:124132 Cannot connect to IPMI host. Error 0x16 Invalid argument

frontend, on the other hand, only shows :
"Cannot connect to IPMI host. Error 0x16 Invalid argument"

it would be helpful, if frontend would also show EINF contents, if available - obviously, they tell more about the problem