I was able to login to my UK Exchange Admin Center a few hours ago and now I get this error I when trying entering the URL:

"Error executing child request for /owa/auth/errorFE.aspx."

I was also receiving this error all the time for the other sites. Please assist!

Are you asking about an on-premises server or Exchange Online?  If it's Exchange Online, I recommend you open a ticket with Microsoft Online Support.  If it's on-premises, please share your service pack and update rollup level of Exchange 2013.

It is on-premise and the version is Version 15.0 (Build 1044.25). Which has the RU7 installed.

What do you mean by "I was also receiving this error all the time for the other sites"?

The servers in UK always worked when I accessed Exchange Admin Console using a UK server url. But servers from other sites, I would recieve the error "Error executing child request for /owa/auth/errorFE.aspx" which is now the same error I'm getting from the UK servers.

Does this thread help?

https://social.technet.microsoft.com/Forums/en-US/33be5553-1b4d-4c39-83b4-be375055970e/exchange-2013-cu6-cas-not-working-across-ad-sites?forum=exchangesvrclients

You might also try updating to CU9.