I have a problem with a calculated Item after rename a host.
I recevie this error:

root@zabbix:/var/log/zabbix# cat zabbix_server.log | grep MIMX240:ifInOctets.COGENT
last("MIMX240:ifInOctets.COGENT")+
last("MIMX240:ifInOctets.COGENT")+
11458:20141219:114800.408 calcitem_parse_expression() functionid:5 function:'last("MIMX240:ifInOctets.COGENT")'
11458:20141219:114800.408 calcitem_evaluate_expression() function:'MIMX240:ifInOctets.COGENT.last()'
11458:20141219:114800.410 Item [AS-DTSRETEIVO:total_in] error: Cannot evaluate function [last()]: item [MIMX240:ifInOctets.COGENT] not found
11477:20141219:114802.895 item [AS-DTSRETEIVO:total_in] became not supported: Cannot evaluate function [last()]: item [MIMX240:ifInOctets.COGENT] not found
11477:20141219:114802.895 query [txnlev:1] [update items set lastclock=1418986080,lastns=410537537,status=3,error='Cannot evaluate function [last()]: item [MIMX240:ifInOctets.COGENT] not found' where itemid=23430;
last("MIMX240:ifInOctets.COGENT")+
last("MIMX240:ifInOctets.COGENT")+
11461:20141219:115030.017 calcitem_parse_expression() functionid:5 function:'last("MIMX240:ifInOctets.COGENT")'
11461:20141219:115030.017 calcitem_evaluate_expression() function:'MIMX240:ifInOctets.COGENT.last()'
11461:20141219:115030.019 Item [AS-DTSRETEIVO:total_in] error: Cannot evaluate function [last()]: item [MIMX240:ifInOctets.COGENT] not found

but the Item ifInOctets.COGENT exist and Zabbix get all information in SNMP.

Why the calulated item don't found ?