Hi, 

Have you changed anything on the storage where previous incrementals may no longer be available? As the Synth Full will need the last full and all the incrementals after.  

A quick resolution will be to run a new tradition full. 

@Eldan the only case this can happen is when the media from which we are trying to read the data is not available or is corrupted.

@Eldan , as per the advice above, we can’t access the Index Cache to complete the Synth Full.

Is the source data on tape?  If so, make sure all of the media is in the library.  If it is on disk, then it is possible the index is corrupted (and you’d be better off opening a support case).