I guess that is just one of the things you've to know, so maybe it helps someone else.

We saw some warnings in our playbook rollouts like

[WARNING]: sftp transfer mechanism failed on []. Use
ANSIBLE_DEBUG=1 to see detailed information

They were actually reported for sftp and scp usage. If you look at the debug output it's not very helpful for the average user, similar if you go to verbose mode with -vvv. The later one at least helped to see parameters passed to sftp and scp, but you still see no error message. But if you set

scp_if_ssh: True


scp_if_ssh: False

you will suddenly see the real error message

fatal: [docker-023]: FAILED! => {"msg": "failed to transfer file to /home/sven/testme.txt /home/sven/
.ansible/tmp/ansible-tmp-1568643306.1439135-27483534812631/source:\n\nunknown option -- A\r\nusage: scp [-346BCpqrv]
[-c cipher] [-F ssh_config] [-i identity_file]\n           [-l limit] [-o ssh_option] [-P port] [-S program] source
... target\n"}

Lesson learned, as long as ansible is running in "smart" mode it will hide all error messages from the user. Now we could figure out that the culprit is the -A for AgentForwarding, which is for obvious reasons not available in sftp and scp. One can move it to group_vars ansible_ssh_extra_args. The best documentation regarding this, beside of the --help output, seems to be the commit message of 3ad9b4cba62707777c3a144677e12ccd913c79a8.