In my case, the cvs client couldn't connect to the server because of authorization issues. The solution was to add an environment variable:
- CVS_RSH which points to the Plink executable (Plink is the one which tries to establish the client/server connection); for example, C:\Program Files\TortoiseCVS\TortoisePlink.exe.
- Right Click on "My Computer" -> Properties -> Select "Advanced" tab -> Click on "Environment Variables" at the bottom of the window -> Add CVS_RSH as a system variable.