I'm having trouble syncing to my remote GitLab repository through the SourceTree user interface. When I issue a 'pull' request, I get an error saying the host key is not cached for this server ...
However, when I issue the command at the Terminal, it appears to work. It populates the known_hosts file under my profile and I can perform a 'git pull' at the command line. The user interface then either hangs or gives this error. I populated my private SSH key *.ppk file following normal protocol 'ssh-keygen -t rsa -C "my.email@myhost.gov"' and re-saved this to a PPK file using PuTTYgen.

I've also properly added my SSK key to my profile on my matching GitLab profile.