I've been using Eclipse with CVS over SSH for some time with no problems.
Today, Eclipse started complaining with:
unknown response received from cvs server
whenever I try to do any CVS operations.
What's causing this issue?
Simple solution!
It turns out that my SSH password had expired, and needed to be changed. I simply logged in to the target server over SSH, changed the password, and then updated my Eclipse CVS configuration to use the new password.