I have an RTMP SIP Gateway license key xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx. But the key does not work. Why?

The license module was changed.
If it’s the latest available build (1650) from here: https://flashphoner.com/download_rsgw/
You should activate the license:

cd /usr/local/WowzaMediaServer/bin
./activation.sh
xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

If you have an older build you should to upgrade your build to the latest one.