Took me a while to figure this one out... hopefully it helps someone else out there.
For the longest time now, my Aeotec Siren 6 was not obeying volume based commands. It was still playing sounds, but stuck at 100% volume. Changing the volume via Rule or manually in the device page yielded the following similar errors in the log:
java.lang.NumberFormatException: null on line 181 (method volumeDown)
java.lang.NumberFormatException: null on line 181 (method setVolume)
So I enabled debug logging on the device page and lo and behold the commands work now. I turned off debug logging to make sure I wasn't crazy and again I get the Java errors. So I guess the only way for the device to function correctly is to leave debug logging enabled for whatever reason.
Debug logging shouldn't be required, but putting that aside, doesn't debug logging automatically turn off after 30 minutes. At least it does on my hub and I haven't found a way to stop that.
Lol.. yeah, it's certainly a neat option.
Some developers give a multiple selection (30 mins, 60 mins, 24 hours, etc.)... followed by an auto-off. Others just have a simple on/off.
I'm not seeing any reason debug logging should matter for this (there never should be, but typos can introduce weird bugs...). However, if you have it enabled, you should be able to show some logging for successful commands. If you try the same ones unsuccessfully, what is the log output for both?
The value of your "Chime Tone" setting might play into this, however. What is that set to? Does it affect anything if you change it?
Additionally, what hub platform version are you running (Settings > Hub Details) and have you ever used a beta version?