I recently noticed errors playing back music. The error messages in the logs said:
MEDIA_ERROR_SERVICE_UNAVAILABLE. If this happens on every track, please check your computer firewall settings or review our FAQ.
In the FAQ under the question "When I ask Alexa to play music from My Media, there is just silence," There is a phrase that says,
"To test that network access is ok, try to access the following URL from a web browser on another computer/mobile/phone on your network: "
I noticed that the IP address listed in the test URL was incorrect. Since the last time I had used mymediaforalexa I had changed the subnet my Unraid server was using and the old subnet was still showing up. This old IP was despite the fact in the docker configuration I had the correct IP address.
I solved the problem by stopping the docker container and then editing the file
/mnt/user/appdata/MyMediaForAlexa/Preferences.xml.
Within the XML tag <UseIP4Address>OLD IP</UseIP4Address> was the old IP address I had previously assigned to MyMediaForAlexa. I edited that file, as well as Preferences.xml.bak which I don't if was necessary but I did it anyway, to the correct IP address.
Then I restarted the docker container and my connectivity problems went away.
I was using a custom IP address on a bridged network, if that makes a difference to anyone else.