How can i fix the Oracle Enerprise Manager error - Agent is blocked?
I suggest you re-sync your agent using the EM GUI ! 
Here is a article showing how this is done !

In short : 

Check your agent status :

./emctl status agent

Oracle Enterprise Manager Cloud Control 12c Release 3
Copyright (c) 1996, 2013 Oracle Corporation. All rights reserved.
---------------------------------------------------------------
Agent Version : 12.1.0.3.0
OMS Version : (unknown)
Protocol Version : 12.1.0.1.0
Agent Home : /em_agent12.3/agent_inst
Agent Binaries : /em_agent12.3/core/12.1.0.3.0
Agent Process ID : 64164
Parent Process ID : 64110
Agent URL : https://bih002:3872/emd/main/
Repository URL : https://dcg023:4900/empbs/upload
Started at : 2013-11-13 15:26:08
Started by user : em_user
Last Reload : (none)
Last successful upload : (none)
Last attempted upload : 2013-11-13 15:26:14
Total Megabytes of XML files uploaded so far : 0
Number of XML files pending upload : 72
Size of XML files pending upload(MB) : 0.11
Available disk space on upload filesystem : 91.16%
Collection Status : Collections enabled
Heartbeat Status : **Agent is blocked**


Next re-sync your agent using the GUI:


Now check you agent status again!