Cheers bud! Was getting frustrated by this since it was all my audio apps.
The frustrating part of this I have to use the stock lockscreen and can't use the one within goodlock but ah well. I'll take it for now.
I had the issue of the spotify controls from the lock screen widget not working.
What resolved the issue for me was:
Settings > Lock Screen > Face Widgets > Turn off the music option
Spotify then worked for me straight away.At a guess I would say there might be some type of resource clash with the default samsung music which I don't use and spotify. So turning that off resolved the issue for me.
I was the same situation as yours, and no tips mentioned here couldn't solve the problem.
Finally I found the solution.
Just uninstall the latest spotify and install old one (such as Ver.8.4.9).