I wanted to try and use DS95 to output to a different program and if I try to use the "Export to Sprite Sheet" function, it looks like it bugs out cause it adds an extra "Assets" folder when it's searching for the file name.
Below you can see the project hierarchy and the error message. I can get it to work if i copy the sprite to a dummy folder that it's trying to search for and then run the sprite sheet operation again. I'm using 2019.1.8f1 and DS95 2019 version (but i tried 2018 and does same thing it seems).
Hope that helps!