This issue is now resolved. Here is the issue and the fix from the website host:
ERROR:
It appears that the server's Mod Security module is preventing the script from executing. It is displaying the following error in the Apache log.
[Wed May 06 14:44:33 2015] [error] [client 67.XX.XXX.XXX] ModSecurity: Output filter: Response body too large (over limit of 2048000, total not specified). [hostname "xxxx.org"] [uri "/administrator/index.php"] [unique_id "VUphCsaIPUYAAFuHbrYAAAA6"]
FIX:
We've made an adjustment to the ModSecurity configuration to your domain to resolve this.