NM, we finally decided the only explanation for the problem must be some form of SQL database corruption. So we searched for available methods to repair the database and discovered WP-DBManager plugin (https://ww.wp.xz.cn/plugins/wp-dbmanager/).
After backing up the SQL database and running the WP-DBManager database repair option the problem with the two subdirectory based WordPress websites appears to have been resolved. FYI for anyone else having this type of problem.
We run out own dedicated server, so we are the hosts…
We have 3.5.2 and the same problem on only 1 out of 38 sites. Unfortunately this bit of code didn’t fix the problem for us. When we click either the top or bottom “Select All” check-boxes.
http://cjst.pw/images/WordPress_2013-07-31_02-02-11.png
http://cjst.pw/images/WordPress_2013-07-31_02-06-11.png
