The IP address should be enclosed in [ and ] :
time="2020-11-18T18:05:32Z" level=debug msg="secret: external: cannot get secret" error="performing request: Get https://fdec:dead:beef::1:443/api/v1/namespaces/ci/secrets/test-drone: invalid URL port \"dead:beef::1:443\"\n" kind=secret name=mysecret thread=8