Concrete 5.6 no longer functioning properly in any browser

Permalink
I have built and edited a number of websites using Concrete5.6 in the past and just done 2 using 5.7 so I know how to use the CMS.
Recently I had difficulty adding a page to one of my older sites that I originally built nearly 2 years ago with no problems. I contacted the Theme designer as I thought it might be a theme issue but he couldn't find out why I couldn't add the page. Another post suggested that they'd had an issue using Firefox as the browser so yesterday I tried using IE and managed to add the page. Today I can log onto my site editor to now complete and edit the new page but as soon as I try to go to the page either via the navigator on the home page or to try to go to sitemap in the Dashboard the screen leaves the editor mode or throws me out back to the login page.
This has now happened in IE, Chrome and Firefox. Why won't Concrete5 work properly any more? Before I was frustrated because I couldn't add a page. Now I have a half finished page that I can't edit that lets the site down. I need this sorted quickly.
I am quickly losing faith in Concrete 5 as it seems to have developed quite a few issues that weren't present 2 years ago.
Thanks

 
RogerM replied on at Permalink Reply
As an update, I just tried again using Firefox and the editor seems to be functioing but it could just be temporarily for all I know. There seems to be no consistency or logic why it works or doesn't in whichever browser.