Answered 404 error in some browsers, but not all ?

Ok, this is the weirdest thing ever.
I have set up a site with translations in a subdirectory of the root.
My main site works fine. The translations are located in subdirectories like mysite.com/en/ and I do not use any special addon for this, they are just 'new' pages in the 'en' container.
The "en" page is actually the homepage of the english section.
When I use my standard firefox browser everything works like a charm. However when I visit the site in chrome or in private mode in firefox, the /en/ page will return a 404 error.
The subpages of this container page (mysite.com/en/pagename.html) work fine.
I tries to circumvent the issue by creating an index page and visiting that in stead of the /en/ page, but this gives the same problem! It is available in my standard browser but not in the other ones.
I have set up friendly url's and assume this is where the problem is generated.
Where do I begin to look?

Does the URL shown in the browser's address bar switch from www to non-www or vice versa when this happens?

What are you using to generate your menu?

pdoMenu appears to have a &previewUnpublished option. This 'shows unpublished resources only for logged-in Manager users with permission to view unpublished'.

It also has a &showUnpublished option. This 'shows unpublished resources to everyone'.

Could one of these have caused the problem when you were not logged in?

I only use standard wayfinder menu's

As the admin user, you have view_unpublished permission. When testing access for another user, be sure you visit the site in a different browser where you're not logged in to the Manager.