HTTPS causes an error: Third party libraries not installed

So I've installed Concrete5 8.5.1 and all seemed fine except if I enter https instead of http, I get a blank screen with only the words:
"Third party libraries not installed. Make sure that composer has required libraries in the concrete/ directory." ssl is enabled on my site, so I'm not exactly sure where I've gone wrong.
I used the tutorial for a multi-site approach because I've used it before and it makes it easier to do upgrades and downgrades because the concrete directory is a symbolic link. I used this tutorial. I thought this worked for me before:

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