Git push to local network repository behind ssh fails

Push to local network server with per-user ssh access (password auth) fails with message:

ssh_askpass: exec(/usr/X11R6/bin/ssh-askpass): No such file or directory

Permission denied, please try again.

CLI push with either system git or embedded git succeeds. Using system git instead of embedded git for SourceTree fails with the same error. 

Note that /usr/X11R6/bin/ssh-askpass does not exist on a stock macOS system.