time machine error: the backup disk image could not be mounted

for the past month or so my automated backups with time machine/time capsule have not been working. Every so often the following error message pops up:

"the backup disk image could not be mounted"

in troubleshooting, i have tried to mount the sparsebundle image directly but without success. from reading around a bit i can see that others have had this problem, and that one solution is to delete the sparsebundles and start again, but that of course means losing all my archived backups, which i would rather avoid.

so just checking if there is anything else i could try first, and wondering what might have caused this problem in the first place

thanks

nick

mac pro 10.5.6