I finally updated daughter's BI4 over the weekend and all went well except I am getting this sound error when I try to set a logon alert sound. Which btw was already configured in BI4 but I wasn't getting this error. This is a headless system that is RDT into. I also shot Ken a email on it.
I still haven't heard back from Ken, but I did find that the sound alerts you had configured in BI4 do not populate the registry UNTIL you update the sound file path and then save it. Once you do this, it will ad many more keys in the Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Perspective Software\Blue Iris\server\users\xxxxxx\OnLogin\0 folder. This along with something new in 18.104.22.168 that keeps sounds from going through a RDT connection. To fix mine I just deleted my on connect alerts for my 3 users, which wasn't so bad.