Open Sans Regular 400 now looks like Thin 300 on client's websites

Okay, this is weird. I think I found the bug, yet I don’t understand it.
I am running a Mac using Linotype FontExplorer X to manage my fonts. As I saw everything thin, locally I had only Open Sans Light active. If I locally only activate Open Sans Bold I see the copy text of the webpage bold. If I deactivate all fonts OR active all fonts I see everything as it should be.
So – as weird as it is – I could say it a local problem. YET, if I look at a website like goo.gl/WBpWUU, which uses Open Sans, too, I don’t have this issue. Regardless what I activate or deacitvate locally on my Mac I see everything fine.

Do you have any Idea, why the Enfold reacts differently than that site? Has it something to do with the way the font is embeded or the way the CSS is written (like ‘font-weight: 400’ or ‘font-weight: normal’)?

Hi John, I deactivated every single Plugin and it didn’t make a difference. But I found a good hint:
https://stackoverflow.com/questions/9420155/locally-installed-ttf-overrides-google-fonts

The fonts in Enfold are embedded like this:

@font-face {
  font-family: 'Open Sans';
  font-style: normal;
  font-weight: 400;
 <strong> src: local('Open Sans'), local('OpenSans'), url(https://fonts.gstatic.com/s/opensans/v13/cJZKeOuBrn4kERxqtaUH3bO3LdcAZYWl9Si6vvxL-qU.woff) format('woff');</strong>
}
@font-face {
  font-family: 'Open Sans';
  font-style: normal;
  font-weight: 600;
  src: local('Open Sans Semibold'), local('OpenSans-Semibold'), url(https://fonts.gstatic.com/s/opensans/v13/MTP_ySUJH_bn48VBG8sNSqRDOzjiPcYnFooOUGCOsRk.woff) format('woff');
}

Note that the 600 font-weight says (‘Open Sans Semibold’). As this is explicit and I didn’t have this activated on my computer, there is no local font to get. But the 400 font-weight only says local(‘Open Sans’). There my computers seems to say: „Well I got an ‘Open Sans’ acitvated here, let’s just take it!” – regardless of the font-weight.
If I locally only activate Thin 300 or Semi-Bold 600 that’s what the website takes for ‘Open Sans’.

Could you check this for yourself? And if this is the issue, if there is a way to fix it.
(The website I showed above, where I don’t have this problem, doesn’t has this line with the local-fonts in the code.)

Thank you!