Installing Extensions not possible

A problem accurred, when I am try to install the extenstions:

Could not query updates site for missing extensions

Unable to read repository at https://update.knime.com/community-contributions/trusted/5.4/content.xml.

CONNECT refused by proxy: HTTP/1.1 407 Proxy Authorization Required

Also if I am copy the URL and browse it, the page can not be found.

Could anyone help?

Generell informations:

• Work PC / Admin rights

• WIN

Thx,

J