chroot, gpg-agent, and Ubuntu 18.04

Solution 1:

It actually was the .gnupg directory that could not be created. The home directory specified in /etc/passwd (/html) is relative during the SFTP session, but understood as absolute path during the login. Therefore...

• Creating a folder .gnupg in chroot+home = /var/www/domain/html did not (!) help.
• Creating a folder /html/.gnupg (.gnupg owned by sftp-user) removed the error message from /syslog.

To take care of further problems (of course, it did not work immediately), I started another sshd daemon in verbose mode and connected to this port .... but that would lead off topic here.

sudo /usr/sbin/sshd -d -p 3321