I've tried the following:
1. Uninstalling Sourcetree. Deleting of /Users/User_name/AppData/Local/Atlassian Downloading and re-installing Sourcetree.
2. Adding
<setting name="AutomaticallyCleanUpDictionaryFiles" serializeAs="String">
<value>True</value>
</setting>
to the user.config file in the Atalassian AppData/Local/... directory
3. Starting as administrator
All suggestions were things that apparently worked at some point or other with users having the same problem.
Thanks for any help or suggestions.