SourceTree fails with the following message when doing anything related with git:

'git something' failed with code 128: fatal: unable to access 'H:\/.config/git/config': Invalid argument

My gitconfig is located in the default path: C:\Users\myusername ; however SourceTree tries to access H:, which is a network drive that is not connected at the moment.

Finally, I found that I have the following environment variable:
HOMEDRIVE=H:

It seems that when trying to read the environment variable HOME, it read HOMEDRIVE instead.

I solved this by adding a new environment variable: HOME=C:\Users\myusername

However this is a bug because it should never read HOMEDRIVE.