After you have "fixed the issue" I appear to be able to enter the add-on the domain gallerywall.me.uk and everything goes o.k.. I can go into the file manager and the appropriate folders are present, I can access them using my ftp client and add files. However the domain does not appear in the add-on domain listing even after waiting 24 hours or more. And trying to access the domain url via a browser returns the "Account Queued" page.
If I subsequently try to re-add the domain I then get the "domain already configured" error. My apologies, I should have explained this better before I realise how difficult it must be to isolate the problem.
Any ideas on my other add-on domain that was working and now stopped? It shows in the add-on domain list and was working fine.
Now when I use the url http://www.museumdisplaycases.co.uk I get a "Server not found" error.
Yet if I use the url http://museumdisplaycases.framework-rd.heliohost.org/ it works fine suggesting it is set up correctly.
I have checked all the domain settings, name servers etc and nothing has changed so I am mystified as why it stopped working. It actually appeared to stop after I re-entered the other add-on domain gallerywall.me.uk following one of your fixes, could they be connected or just coincidence?