For a long time, I had major issues with sending commands via RFXtrx433 under Domoticz (I think I had the problem with my brief encounter with OpenHAB also.)
I was unable to figure out why, save for the fact that it worked when I first got the RFXtrx433 and tried Domoticz – I therefore assumed the error was “in software”, either with adding the device or a bug in Domoticz.
I recently did some searching on a somewhat-similar issue, and I found a hint on a Homeseer-forum, that if the wrong firmware is loaded, transmit won’t work.
I went straight to www.rfxcom.com and downloaded what I assumed was the correct firmware, making sure it was the type 1 since that was one of the caveats the blogpost noted, and flashed that via Domoticz.
(note, triple-check if that’s what you also need, read the included .txt-file in the firmware-zip.)
After the flash, suddenly it just worked.
I guess I did a bad flash or used the wrong firmware once.
The only error I ever got was: “wrong command received from application” in Domoticz.
If you got that one, searched around and found nothing – you might just need a new firmware.