I had this problem back on 11d, where about 2/3 of the time doing IS backups of a Exchange 2007 SP1 cluster, the first storage group wouldn't backup. The job would complete successfully, no errors. Specifics are here.

Well, upgraded to 12, and the problem went away. I had successful backups for over a week straight. Then yesterday, I applied these two hotfixes:
300287 (X64)
300290 (X64)

Last night, again, the first IS did not backup. No error. The only trace something went wrong (other than the amount backed up) was a message in the backup summary saying 'This backup set may not contain any data', and under Backup Set Detail Information, it makes no reference to the consistency checks.

Anyone else have this problem rear its ugly head again?