Hey friend! I appreciate hearing that this project inspired you to explore a musical side of yourself that you haven't connected with in some time. Music is really interesting in the way that we can recall with perfect clarity a tune we heard a decade ago, while at the same time being hardly able to remember what we had for dinner last night. Important to remember that the musical and creative sides of ourselves are always there, even if we haven't had a chance to consciously and intentionally interact with those parts of ourselves for some time.
As for the troubleshooting question, I'm thinking this might be something that I patched at some point down the road but never made a formal announcement for. Based on the description you provide (assuming it's a copy paste) what's missing is as follows in bold and underlined:
"images/Buttons/config/temp/galalexisleepoverh.webp"
In my working file, that is located at line 12498 of the file called screens.rpy. I don't know what would happen if you updated that line of code in a live, unpackaged version of the game, but conceivably that would solve the issue. Of course that could also mess it all up irreparably. I'm not knowledgeable enough about coding to do much besides follow basic instructions on ren'py forums!
Hope that helps, and if not, at least by the time v0.9 is released the error should disappear.