If you open eZ Studio and click on a link on the content which points to another siteaccess, the siteacess will be appended to the URL and you will have a 404.

Steps to Reproduce

Assume your site is hosted at http://1-5.ez and you have three siteaccesses (site, some, other).

• You must have somewhere in your content, a relative link to other siteaccess (a menu, for example)

  <a href="other">Other</a>

• Open eZ Studio to edit your landing page:
  http://1-5.ez/ez#/studio/insite/%2Fsite%2FMypage
• Click on the relative link in the content
  http://1-5.ez/ez#/studio/insite/%2Fsite%2Fother

eZ Studio left your landingpage and add the siteaccess to the url, breaking the editor.