cannot use sourcetree due to illegible error message

Any attempt to clone or checkout a branch using Stash 'checkout with Sourcetree' or 'Clone with Sourcetree' results in an error message on the Sourcetree URL Actions window.
Same if I try to start the clone via Sourcetree menu options.

The error message says "This is not a valid source path/URL", with a details button next to it. When I click details, I get a truncated message "fatal: Cannot change to 'C:/Users/charlie.stott.ECC/..': Permission denied"

As you see, this message contains a truncated parameter and I cannot expand the window or message box to see the full message.

I have removed all mentions of the partial path from my settings.

I have tried system and embedded versions of git and mercurial, enabled them, and relocated the global ignore lists.

When I try to clone from sourcetree menus, I get the same error and details button, and same details shown when i click the button. When I try to change the URL (trying ssh://.. http://.. https://..), the message flashes to ... then gives the same error.

Please tell me what sourcetree is looking for down this path and how I can fix this problem.

Sourcetree is being used by my entire team, but I cannot get past this error, neither can my sysadmin find any problem.

I have searched for a solution and found many people with similar issues, but no resolution, other than reinstall (which I have tried) and pointers to ssh and git settings.

I can use git on the command line with or without ssh. This implies that stash, git and ssh are all working just fine.

I am sure it is a simple matter, but without knowing the full path that is problematic, I cannot find any way to fix myself.