Outlook 2013: Cannot sign or encrypt messages

Question

I've done clean install of Windows 8.1 and forgot to copy my certificate. Luckily I have offsite backup so I imported the certificate into path
"Certificates\Personal\Certificates" in MMC console (under my user account as well as computer account).
But I cannot sign or encrypt messages in Outlook 2013. Outlook says "Outlook had problems encrypting this message because the following recipients had missing or invalid certificates (my e-mail address here)".
Moreover I cannot see incoming encrypted messages, Outlook says "Sorry, we're having trouble opening this item. This could be temporary, but if you see it again you might want to restart Outlook. Your Digital ID name cannot be found by the underlying
security system."
I checked E-mail Security in Trust Center and everything seems to be OK, my certificate/DigitalID is installed, enabled  and valid. I also have imported colleague's valid certificate in
"Certificates\Trusted People\Certificates".
I have Audit Failure in Event Log which says:
Cryptographic operation.
Subject:
Security ID: homepc\username
Account Name: username
Account Domain: homepc
Logon ID: 0x2C5DD
Cryptographic Parameters:
Provider Name: Microsoft Software Key Storage Provider
Algorithm Name: RSA
Key Name: {1F8C5EBB-F54B-4F42-96FA-A02D8B5F9C3D}
Key Type: User key.
Cryptographic Operation:
Operation: Open Key.
Return Code: 0x80090016

Answer 1

Hi,
Please try to perform the following steps:
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps
carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry,
http://windows.microsoft.com/en-US/windows7/Back-up-the-registry
1. Win + R, type regedit, and then click
OK.
2. Locate the following registry key:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\15.0\Outlook\Security
3. If a Security registry key does not exist, create a new key. To do this, follow these steps:
a. Click the following registry key:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\15.0\Outlook
b. On the Edit menu, point to New, and then click
Key.
c. Type Security, and then press ENTER.
4. Right-click the Security key, click New, and then click
DWORD Value.
5. Type SupressNameChecks, and then press ENTER.
6. On the Edit menu, click Modify.
7. Type 1, and then click OK.
8. On the Registry menu, click Exit.
Regards,
Melon Chen
TechNet Community Support