Joomla Content Editor Acces restriction

i’m having another issue with Joomla (latest Version). I’m working with my dad on a websites (on an own windows-Server) and for me everything works fine. As for him: we both use the JCE but when he tries to embed pictures or set hyperlinks every time he opens the window for doing so he gets a “access to this resource is restricted”

So far so good, here is what we tried:

-Setting the rights of course. Folder is fully accessible, we are both Super. Still only works for me.

-Using another editor: works….because they don’t have the nice managing system as the JCE

-When he is logging in from another mashine (physically or remotely): works.

-when he is logging in from another account within his mashine (MacBook Pro): works.

-Logging in to another website of his own and use the JCE there: works, even from his own account on his own mashine

-using another Browser: does not work (no matter if chrome, firefox or opera… (default he uses safari))

-using another Joomla-account: does not work, even if he uses mine.

Conclusion: the Problem only exists on this one website from his own mashine with his own mashine-account.

So there are some workarounds (another account, another Content-editor) but these would not change the existing problem (and he does not want to use another editor). I don’t now what to do anymore, do you have any ideas?