KAZです。

原因が分かってきました。
今回検証にPostgreSQLを使用しているのですが、それが原因かもしれません。
下記にログを載せますが、ログの1行が255Byte以上だと「value too long for type character varying(255)」を出力しzabbix serverがダウンするようです。

MySQLなら大丈夫かもしれません…

linuxのzabbix agentで検証時のzabbix_serverのログ

2216:20090220:121737 [Z3005] Query failed: [0] PGRES_FATAL_ERROR:ERROR: value too long for type character varying(255)
[update items set nextcheck=1235099886,prevvalue=lastvalue,lastvalue='0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345',lastclock=1235099856,lastlogsize=262 where itemid=22265]
2216:20090220:121737 [Z3005] Query failed: [0] PGRES_FATAL_ERROR:ERROR: current transaction is aborted, commands ignored until end of transaction block
[select distinct function,parameter,itemid,lastvalue from functions where itemid=22265]
2196:20090220:121737 One child process died. Exiting ...
2196:20090220:121739 ZABBIX Server stopped. ZABBIX 1.6.2.

Windowsのzabbix agentで検証時のzabbix_serverのログ

WARNING: nonstandard use of \\ in a string literal
LINE 1: ...i.type in (2,7) and ((h.host='PITS130' and i.key_='log[d:\\z...
^
HINT: Use the escape string syntax for backslashes, e.g., E'\\'.
1987:20090220:121429 [Z3005] Query failed: [0] PGRES_FATAL_ERROR:ERROR: value too long for type character varying(255)
[update items set nextcheck=1235099699,prevvalue=lastvalue,lastvalue='0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345',lastclock=1235099669,lastlogsize=266 where itemid=22461]
1987:20090220:121429 [Z3005] Query failed: [0] PGRES_FATAL_ERROR:ERROR: current transaction is aborted, commands ignored until end of transaction block
[select distinct function,parameter,itemid,lastvalue from functions where itemid=22461]
1979:20090220:121429 One child process died. Exiting ...
1979:20090220:121431 ZABBIX Server stopped. ZABBIX 1.6.2.