I heard this from a developer that if the SEND addon isn't working on one char, it's perhaps because they were not logged in fairly recently of each other. So my suggestion is to login both your chars in around the same time. And of course, make sure that addon is loaded on both. This worked for me when using multiple chars.
Keal is correct. It is a known glitch, and not sure there is a way tofix it. Something to do with the IPC messenger just failing to start working some how I think? Usually this happens if you log mules on at different time. Simply re-log accounts back on together and it will fix it.