Load control template file /_controltemplates/15/lms2reaBaseEnrollmentFields.ascx failed: The expression prefix 'ETResources' was not recognized. Please correct the prefix or register the prefix in the <expressionBuilders> section of configuration.
The error appears when a request is made to web applications where EFI solutions are not deployed, and a special web.config modification is required to load lms2reaBaseEnrollmentFields.ascx. The modification is available when appropriate web application feature is activated (REM Web Application Extender).
The initiator of such processing is SharePoint, and in other words we cannot manage when and where templates files should be loaded. These template files are global for the farm not for particular web application. So every web application loads the same set of control template files (just all available control templates files and it doesn't matter which solution provides them), caches them and then users from cache. Microsoft has a similar issue with their control, see more at http://support.microsoft.com/kb/2481844
This error does not block any REM functionality and is visible for server administrators only. For the time being the best workaround is ignore the error (other than deploying solutions to all the web applications).