Temporarily unable to find filters located in Windows PATH

We make use of git-media (https://github.com/vitei/git-media) to store large files outside of the git object database and retrieve them automatically in the clean/smudge filters. Usually, this works fine, but occasionally SourceTree reports that it couldn't find the `git-media` executable (see attached screenshot).

Discarding the message and then refreshing usually works. Sometimes it takes a few refreshes. These failed filters can cause unwanted side-effects, such as marking files as modified when in fact they haven't been, requiring us to manually discard the changes – so the issue is worse than just an annoying error message.

It's as if the PATH environment variable has been temporarily modified, or for some reason git/SourceTree have temporarily "forgotten" the location of git-media. I have confirmed that the executable is locatable in both the system and the user PATH environment variables. Unfortunately the problem is usually so fleeting that further research has proved difficult – by the time we click through the error message the problem has resolved itself. I am fairly certain that the problem is not with `git-media` specifically, but would happen when running any filter that requires git to run an executable/script located in PATH.