Errors on Exchange Backup

Hi, sorry for the long post but I'm trying to get everything straight in the below explanation. 

We recently created a new domain user account to run the backups (we had been using the domain administrator account which was causing other problems on one of our domain controllers).

Since this we have been having errors relating to permissions on the exchange server, the error is:

Backup
Directory
Unknown
 was not found, or could not be accessed.
None of the files or subdirectories contained within will be backed up.
V-79-57344-5889 - The object was not found, or could not be accessed (Exchange backup).

(shown at the end of the log in the errors section accompanied by):

Unable to create the temporary files that are needed to extract granular data for future restore. Make sure that the credentials used to perform the operation have write access to the BIN folder on the application server, or to the DATA subfolder if it exists.
Directory <Unknown> was not found, or could not be accessed.
None of the files or subdirectories contained within will be backed up.
V-79-57344-5889 - The object was not found, or could not be accessed (Exchange backup).

(Shown in the mail server section of the log).

I have created a domain group called backup users (which the account being used to run the backups is part of) and given this full control over C:\Program Files\Exchsrvr\bin on the mail server but I am still getting this error.

The credentials check returns no errors.

I'm going to add the backup user to the domain admins group to see if this solves the problem (when I runt he backup using my log on, which is a domain admin account it's fine) but that seems a bit like using a sledge hammer to crack a nut....

Is there any other folder that I need to add permissions to?

Many thanks

Matt