Backup Failure At Only One Partition Of Client

The nightly backup of my client with lightsout failed.
I've configured the backup for this client in WHS2011 console for the partitions c,d,f,g,h,i and k.
The backup failed for the partitions d and k. These partitions are both on the same disk together with systemdrive c.
The other partitions were succesful.
I tried manual backup for all these partitions and now only partition k failed.
I've checked the partition 'k' for failures in the filesystem and for bad sectors but this seems to be okay.
In the logbook of the server I found:

Back-up of K:\ failed
Event ID: 262 Qualifiers: 53247 Level:2 Task :1 and
Event ID: 269 Qualifiers: 53247 Level:2 Task :1 Reason :7
I really don't know what this means
What else can I check to solve this backup problem?
Any help will be appreciated