OK the problem was that after converting the original EC key to PKCS8 the header no longed contained the "EC" keyword. So instead of BEGIN EC PRIVATE KEY only BEGIN PRIVATE KEY remained. Logstash therefore handled the key as RSA. So after key conversion I used sed:
sed -i -e 's/BEGIN PRIVATE/BEGIN EC PRIVATE/g' /usr/share/logstash/config/tls.pkcs8.key
sed -i -e 's/END PRIVATE/END EC PRIVATE/g' /usr/share/logstash/config/tls.pkcs8.key