"Access Denied" when saving Custom CSS

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Hello! Long time user, first time poster here.

I am working on a C5 site on a hosting provider I've never used before, and for some odd reason I am unable to save any custom CSS on this site (under the "Customize Theme" section). I am logged in as the Super Admin, and I've done this 10+ times on other sites with no issue.

When I click save on the custom CSS window, I get an error box that says "Access Denied". That's it. Is this a file permission error or something? I am able to edit the theme's colors without any issue.

Thank you!
-Victor

ViktorNova