SourceTree unusable on startup ('ssh-agent' failed with code -1: System.NullReferenceException)

This appears to be a regression of SRCTREEWIN-4506.

Every second time I open SourceTree, I get the initial error

'ssh-agent' failed with code -1:

System.NullReferenceException: Object reference not set to an instance of an object.

()

Followed by several (about 3) of these errors:

'git tag' failed with code -1:

System.ComponentModel.Win32Exception: The system cannot find the file specified

(<path to my local repository>)

An equal number of errors which are idential except the command used is 'git status' not 'git tag'.

Attempting to load the branch view (by clicking the master branch) results in the same error for 'git log', and selecting the working copy again results in 'git tag' and 'git status' errors. Any action within SourceTree which would trigger an update of the relevant views causes the errors to be rethrown.

Restarting SourceTree works, as, like I mentioned, it only happens every second time SourceTree is opened (i.e. after a successfully started instance of the program is closed).

This affects versions going back quite a while, but I don't have specific version numbers. In the past it would crash on startup instead (still every second run), and the BugSplat reporter would open, this was fixed sometime in the last 6 months or so, I would estimate.