Using then file selector on the General tab of Tools->Options, results in a path containing windows path separators, ie *\ *. OpenSSH expects unix path separators, ie */*, thus the configured key is not used by OpenSSH on startup.

Immediately after adding a given key it is used as expected by OpenSSH, but is ignored after a restart of SourceTree.

Manually changing the separators solves the problem, but the messages received while the path is in the incorrect format make it very difficult to pinpoint the root cause of the issue.