Wrong font (Revue) in Apple Mail and WhatsApp Web - webkit problem?

Hey guys,

I've been trying to solve this problem for so long now: when using the WhatsApp Web application on my Mac it uses the wrong font. (After some research I found out it's called "Revue", see pic below)

I literally tried everything I can think of: uninstall/reinstall the app, deleting font caches, reinstalling fonts, solving write/read permissions, etc.

Yesterday, I got an email from Apple if I want to join the Beta program, and that's the first time this font problem appeared somewhere else (in Mail) on my Mac, too. (see pic below)

Now as I saw the font problem appearing in Mail, it got me thinking. So I opened the email with TextEdit and checked the spot where it switches to the wrong font. This is what I found:

font-family:system,-apple-system,-webkit-system-font,SFNSText,'Segoe UI','Helvetica Neue',Helvetica,Arial,sans-serif;

So I think this "wrong font problem" has actually something to do with Apple's integrated webkit, because as far as I know WhatsApp Web is using it, too.

But this is where my wisdom ends. Is there a possibility to reinstall the webkit, or some other way to get rid of this wrong font?
Two more important pieces of information:

• this problem was already there before I updated from Sierra to High Sierra, so updating didn't help
• I don't have this "Revue" font anywhere on my computer

Thanks in advance!