Strange that I (and others) didn't run into this previously. From what I've been reading on Ren'Py help boards, it looks like the "switch could not choose a displayable" error you're seeing a lot is a coding issue. Apparently it happens with a switch (a programming statement that goes through a series of possible conditions and picks the appropriate outcome) either pointing to something not yet defined or looking for a condition that can't exist. The game therefore doesn't know what image to try to display as a result of the current condition or the undefined variable.
But I wouldn't think it's possible for others to avoid a bug like that. Um, were you loading an old save? Maybe there's a bug in what you currently have. I would create a copy of your save as a backup, but then you might try F95's
Ren'Py Save Editor. First, try opening
renpy/common/00start.rpy in an editor (Notepad++ might work; otherwise you might try the
Ren'Py Resources Viewer) and see what variables might be present. Then, with your save opened in the Save Editor, see if you can find the variable (there's a search box in the top right to narrow variables down). If you see something there, maybe I could compare my own save's value with yours to see if something's amiss.
Note: If you use the Resources Viewer to see the file, you'll want to select just the main
Happy Summer folder. The program will then see a
renpy folder and you can use folder navigation on the left to drill down and find the file.