The Error 26276 was driving me nuts.
I issued a new SMIME Certificate, clicked on the link "Click & Install Comodo Email Certificate" in the confirmation Mail from Comodo. Download started and I tried to import this Certificate CollectCCC.p7s to the keychain.
But the macOS Keychain returned Error 26276: An error has occured.
The Solution is pure simple: Use firefox!
Copy the link from the email and open it in Firefox.
Now go to "Privacy & Security" in the firefox settings. Scroll down to "Certificates" and click "Show Certificates". Switch to the Tab "Your Certificates". There is your new Cert.
Now you can Export it with "Save ..." to your harddisk as .p12 File, which can be imported to your keychain with a doubleclick.
That's it.