You have installed a Deep Security Agent (DSA) on a Windows OS with Core MSI Package. When you attempt to activate the DSA via Windows command line prompt (CMD), a certificate error similar to the following will show:
Path is: C:\ProgramData\Trend Micro\Deep Security Agent\dsa_core\ds_agent.crt Error is: error:02001005:system library:fopen:Input/output error [fopen('C:\ProgramData\Trend Micro\Deep Security Agent\dsa_core\ds_agent.crt','r')] sslCtx:UseCredentials() failed - error:2006D002:BIO routines:BIO_new_file:system lib 
The error probably occurs because the Windows command line prompt was started with insufficient permissions. To fix this, run the Windows command line prompt with administrator rights. For the procedure, refer to this Microsoft article.
If you are performing an agent-initiated activation (AIA), the dsa.exe called by dsa_control needs to access the agent's certificate file located under C:\ProgramData\Trend Micro\Deep Security Agent\dsa_core\ds_agent.crt.
Starting from Windows 2008 R2 and above, the local administrator account and the user accounts under Administrators group have different permissions. The new Admin Approval Mode (AAM) of Windows 2008 gives Administrators group users (except the local administrator) only a standard access token to folders first. Therefore, clicking C:\ProgramData\Trend Micro\Deep Security Agent\dsa_core (which is owned by Administrator) will trigger the User Access Control (UAC) to ask for permission. This causes the dsa.exe process started by the user account belonging to Administrators group to get an access denied prompt.
To prevent the issue, open the command prompt as Administrator and perform the activation.
To resolve the issue, completely disable the Admin Approval Mode:
- Navigate to Computer Configuration > Windows Settings > Security Settings.
- Click Local Policies, then click Security Options.
- Set the policy settings in the following order:
- User Account Control: Admin Approval Mode for the Built-in Administrator account : Disabled
- User Account Control: Behaviour of the elevation prompt for administrators in Admin Approval Mode : Elevate without prompting
- User Account Control: Run all administrators in Admin Approval Mode : Disabled
- Reboot the machine.