The default binds.conf file has the following line in the [compose::editor] section:
$ex = <C-x>
With this line present, pressing Ctrl+x when composing a new message brings up the aerc command line. However, changing this line to other Ctrl combinations does work. For example, if I change the line to:
$ex = <C-i>
then pressing Ctrl+i does not bring up the aerc command line. (Pressing Ctrl+x doesn't either, so the change does have some effect). The bug does not impact all special values; and other function keys continue to work.
This bug could be fairly frustrating for aerc users who use emacs (or emacsclient) as their $EDITOR, since is an important keybinding that people likely will want to avoid shadowing. (Personally, however, I'm happy using a binding).
I tested this on the latest version of aerc available in the git repo, and on versions 0.4.0, 0.3.0, and 0.1.0; the behavior was the same in all cases. I also asked on #aerc, and another user was able to reproduce the bug.
Please let me know if I can provide any other information.
Best regards, Daniel