An error appears on the zabbix_server.log after creating a discovery rules within the Configuration > Hosts component. The discovery rule in itself does not run and the following entry is repeated in the log:

4575:20150113:121431.891 [Z3003] no connection to the database
4575:20150113:121431.891 invalid discovery rule ID [23682]

Where the rule ID corresponds to the items ID within mySQL database that pointed to the Discovery Rules created manually (not applied through template). Upon setting the debug level to debug, I get the following log entries:

8878:20150114:095904.268 In lld_process_discovery_rule() itemid:23693
8878:20150114:095904.268 query [txnlev:0] [select hostid,key_,state,evaltype,formula,error,lifetime from items where itemid=23693]
8878:20150114:095904.268 [Z3003] no connection to the database
8878:20150114:095904.268 invalid discovery rule ID [23693]
8878:20150114:095904.268 End of lld_process_discovery_rule()

After deleting the discovery rule, the error message went away. I'm able to recreate the error in the same manner with a different discovery setting. Discovery rules that are applied to the host using a template does not cause the same problem.