BTDT, see https://www-secure.symantec.com/connect/forums/enormous-file-caterrorlog . Took me more of two months of work with Symantec support to sort out. In fact it is very simple.
The root cause of the problem is this: Backup Exec has a database table of files to be deleted. Every five minutes, it tries to delete every file in this table and removes those entries for which it succeeded. If it fails, the entry stays on the table and deletion is retried five minutes later.
Unfortunately, this also happens if deletion fails because the file does not exist. So if, for any reason, a non-existent file is listed on that table then Backup Exec will infinitely try again every five minutes to delete that non-existent file, each time logging the message you're seeing in CatError.log. "Error Code -536836945" of course meaning "File does not exist".
The solution according to Symantec support is to manually delete the entries in question from the database table, which of course only a Symantec support engineer should do. So you'll need to open a support call of your own. My case says "Solution: TECH216668" but I cannot see a KB article with that ID, so quoting that may or may not help shorten the time to a solution for you.
(My proposed solution, treating the result "file does not exist" the same as "successfully deleted" and removing the entry from the table, did not find grace with Symantec engineering.)
