I recently updated my WordPress installs to 3.5.1. I am a hosting reseller with over 20 websites on this server. These WordPress installations have been functioning for several months, each with updates to plug-ins and the WordPress core as they became available without any issue. All the installations in root directories are still working correctly following the update. This indicates to me that there is not issue with the host's security settings.
The installations I have in subdirectories return an error identical to the one above with some slight variation dependent on what change I am attempting to complete in the dashboard.
Can you provide me with information on how to resolve this issue as I often install WordPress in a subdirectory when the site is under development.