I think the problem was bash on the server: every non-interactive ssh login was printing error messages to stderr, and for whatever reason TortoisePlink (the ssh client included with Tortoise) could not deal with these. I am not absolutely certain that this explanation is correct, but here is my reasoning, based on trying various combinations of svn and ssh clients on Windows:
- cygwin svn / cygwin ssh: worked fine (the error messages were visible, but ignored, and the svn operation worked)
- collabnet svn / TortoisePlink: "network connection closed unexpectedly"
- collabnet svn / cygwin ssh: worked fine!
Once I changed my login shell to eliminate the error messages, the latest version of Tortoise (1.6.5) was able to connect to a remote svn+ssh repository.
Now, if I could somehow get back the hours I spent figuring this out... :-)