Repair Time Machine sparsebundle that will no longer mount

Question

I messed up my Time Machine backup in some way. I'm no longer able to mount the sparsebundle file as I get an error stating that there are no mountable filesystems.

I used the hdiutil command to attach the sparsebundle file:

hdiutil attach -nomount -readwrite flattop.sparsebundle

which resulted in the following /dev/ devices:

/dev/disk2              Apple_partition_scheme
/dev/disk2s1            Apple_partition_map
/dev/disk2s2            Apple_HFSX

Afterwards I ran the fsch_hfs command to check the main volume (/dev/disk2s2):

fsck_hfs -drf /dev/disk2s2

This resulted in a notice that the Time Machine Backups volume is corrupt and needed to be repaired:

Unable to open block device /dev/disk2s2: Permission deniedjournal_replay(/dev/disk2s2) returned 13
** /dev/rdisk2s2 (NO WRITE)
    Using cacheBlockSize=32K cacheTotalBlock=32768 cacheSize=1048576K.
   Executing fsck_hfs (version diskdev_cmds-540.1~34).
Non-empty journal:  start = 66310144, end = 94912512
   Journal need to be replayed but volume is read-only
** Checking Journaled HFS Plus volume.
** Detected a case-sensitive volume.
   The volume name is Time Machine Backups
** Checking extents overflow file.
   Unused node is not erased (node = 3568)
   Unused node is not erased (node = 3574)
   Unused node is not erased (node = 3575)
** Checking catalog file.
** The volume Time Machine Backups was found corrupt and needs to be repaired.
    volume type is pure HFS+ 
    primary MDB is at block 0 0x00 
    alternate MDB is at block 0 0x00 
    primary VHB is at block 2 0x02 
    alternate VHB is at block 2865568974 0xaacd1cce 
    sector size = 512 0x200 
    VolumeObject flags = 0x07 
    total sectors for volume = 2865568976 0xaacd1cd0 
    total sectors for embedded volume = 0 0x00 

As you can see there is also an error saying "Unable to open block device /dev/disk2s2: Permission deniedjournal_replay(/dev/disk2s2) returned 13".

I thought this might be due to not running the fsck_hfs command as a su, so I tried it with sudo but this had the same result.

My sparsebundle file is on a Synology DS408 NAS and has been running without a problem for about 2 years now :(

Anyone an idea how to take this further?

Kind regards, Niels R.

UPDATE: As I suspected while writing this question I probably have a problem with read/write permissions. I now see the volume appearing in the Disk Utility and when I click on "Verify" I get the following output:

Verifying volume “Time Machine Backups”
Checking file systemJournal need to be replayed but volume is read-only
Checking Journaled HFS Plus volume.
Detected a case-sensitive volume.
Checking extents overflow file.
Unused node is not erased (node = 3568)
Checking catalog file.
Keys out of order
The volume Time Machine Backups was found corrupt and needs to be repaired.
Error: This disk needs to be repaired. Click Repair Disk.

Can I simply chmod the sparsebundle file to set the right permissions?