Yesterday for some reason the keyserver pool was unresponsive. --> --recv-keys just waited indefinitely. Additional problem occurs with Enigmail, which just shows a progress bar, waiting. There is no API access to terminate gpg.exe from there, so it remains open in memory.
I think it'd be best to add a timeout-option to --recv-keys, so Enigmail will get a fitting error message in such a case and gpg.exe terminates correctly.