Could you check whether the "<someDriveLetter:>\Kaseya\WebPages\BackupExec\ASPx\Web.config" file contains the following information in it or not?
<trust level="Full" />
If this file contains trust level other than "Full", try updating to "Full" and save it using Notepad editor.
If you feel this file is corrupted or completely missing the above tag, you may backup this file in a different folder, DELETE the original web.config from 'ASPx' folder and then run 'repair' of BE plug-in which should ideally deploy a fresh copy of the web.config file.
Regards,
Ramesh Bupathy | Symantec