If unexpected things are read into the client's buffer during login, it assumes that the server it's trying to connect to does not run ThinLinc. However, it may just be the case that something is writing output before thinlinc-login gets a chance to do its thing - for example, if someone echos something in .bashrc.
We should see if we can somehow try to sanitize the input, or at least log what was actually received. I imagine this will have to be a "best effort" solution.
See issue 14025.
This will be done as part of bug 3183.
Fixed in r26736.
Works great, output is logged to tlclient.log.