Sorry guys, just posted this in a new topic so will delete and continue here. Having this issue too - was there ever a fix found for this? This site is running DMS v1, and I am however NOT locked out of the editor, as I have changed the slug with iThemes security. The error is just present on /wp-admin URL and when installing plugins etc.
Disable Error Notices is also checked in the editor. debug is not however.
With my naivety it seems to be related to the LESS processing on load?
I have changed the login slug using iThemes Security, but then I have done that on other installs too without the same issue. I seem to be getting various versions of this error too upon mostly all php executions e.g. trashing a comment, installing a plugin, etc.
DMS Debug Info
PHP Magic Quotes gpc
Yes! Deprecated as of PHP 5.3 and removed in PHP 5.4