When this error is reported, it would be nice if it told me WHICH server was out of date.
There is a local server and a remote server. If the hostname was printed, there would be no confusion.
In the context of rsync, the "server" refers to the remote server. This is well-established terminology, but it ought to be stated explicitly in the man page.
I added a mention of the terms client and server as they are used by rsync to the opening section of the manpage.