This bug does exist ever since the UI update and finally I was able to replicate it:

Simply having the language settings at German will completelly remove the tab that allows you to accept invites, trades, to go back to the lobby room etc if you are inside a mission or in clockworks.

It is not that it is just minimized and my interface is locked. The part of the interface that handles invites and trades is just gone for good and only comes back with english language settings for some weird reason.

Reproducing:
Set your language to German and enter a mission. The tab to handle invites and trade is just gone.
Set the language to English and reenter the mission and the tab is available.