I have read a few posts on this issue but thought it worth a new post to see if a conclusive response/solution will be forthcoming.

Customer has 2 servers both running BE 12.5. One was updated to SP4 and immediately started erroring during the attempt to backup the System State. Whilst other posts indicated a Hot Fix that their first server did have installed, having tied all the other suggested fixes (respecifying the logion account to include the domain etc) I uninstalled SP4 and now have a working backup once more.

Now whilst recent experience with Symantec (SEPM) has left me slightly bitter and twisted and certainly no longer the Symantec flag waver I was, I do still look forward to a positive suggestion as to why SP4 is causing this problem. Or in the unlikely event that Symantec don't provide sucha  response, perhaps someone else out there has worked out a fix.