A client of mine was using TortoiseSVN and was having difficulty committing a change to the repository. Below is a similar error message to the one he was getting.
svn: Commit failed (details follow):svn: MKACTIVITY of ‘/foobar/!svn/act/1f177b34-1601-0010-84ad-c180bd3a7ab5′: 403 Forbidden (http:/svn.foobar.com)
The issue is that Windows is case INSENSITIVE but SVN is case SENSITIVE.
In TortoiseSVN right click the checked out folder and 'Relocate' to the case SENSITIVE path.
IE https://contoso.com/repo -> https://contoso.com/REPO
We were using TortoiseSVN and VisualSVN Server.