According to (info "(elisp) Key Binding Conventions")

     Don’t define ‘C-c LETTER’ as a key in Lisp programs.  Sequences
     consisting of ‘C-c’ and a letter (either upper or lower case; ASCII
     or non-ASCII) are reserved for users; they are the *only* sequences
     reserved for users, so do not block them.

"Debian Bug Tracking System" <owner@bugs.debian.org> writes:

The offending file is "org-secretary.el", which rebinds several keys
reserved for users. It seems to have registered "join" as an autoloaded
function from org-secretary.el, which sounds like a pretty bad choice of
name. I think I ran it by mistake when attempting to run #'join-line.