Choose an uncertified key, right-click "Certify", check the key name and "I have verified the fingerprint", click "Next", choose "Certify for everyone to see" and "Send certified certificate to server afterwards", then click "Certify"
The first time this is done it works and a warning window is shown about being sure to have a revocation cert.
Repeat this for a second key and it will fail. Symptom is that the window will remain with "Cancel" available and "Finish" greyed out. Eventually, an error message may be displayed:
An error occurred while trying to export OpenPGP certificates.
The output from C:\Program Files (x86)\GnuPG\bin\gpg.exe was: gpg: sending key D075EF849BC26AB3 to hkp://keys.gnupg.net gpg: keyserver send failed: Unknown error gpg: keyserver send failed: Unknown error
Shutdown Kleopatra (right-click on task bar icon) and restart it, and the following error message is shown:
"The Kleopatra GPG UI Server Module could not be initialized. The error given was: Could not bind to socket: Unknown error"
Self test after this shows "multiple instances" of "UIServer Connectivity", and "it seems another Kleopatra is running (with process-id <id>)"
Shutdown both instances of Kleopatra and restart Kleopatra again.
Repeat test, still unable to certify.
Shutdown Kleopatra once more and check task manager. Multiple instances of "GnuPG's OpenPGP tool (32-bit)" are running.
Kill all instances of that as well as "GnuPG's private key daemon" and "GnuPG's smartcard daemon". Restart Kleopatra.
"OpenPGP tool" is shown in task manager and then shuts down, leaving only "private key daemon".
One of the keys that could not be certified is now shown as certified - the first one that failed.
Certify a key, Kleopatra prompts for passphrase and certification is successful. Close window.
After this, Task Manager shows one instance of "GnuPG's OpenPGP tool" still running, which closes after a while with a message "OpenPGP keys exported successfully".
Subsequent attempts are much faster with the "OpenPGP keys exported successfully" window showing before the certify window can be closed.
Conclusion: There is some processing during the first certify action after Kleopatra has been started that takes a decent (10 seconds or longer) time, and if a second certify action is started before the "exported successfully" window is shown, multiple instances of "OpenPGP tool" are started which get in each other's way.
This might be difficult to re-create. Let me know if I can clarify anything or you want additional tests.