Backup failure

ATIH12 has been working OK (only started a month ago) but then last week my daily backups (to the ASZ) started having the yellow triangle warning sign and now my backups fail completely (red cross).

I wondered if this was due to the size of the ASZ which I can see is now about 65% full, but then could not manage to increase the size; I get a comment that it is set at maximum size (300GB). I do not think that this is likely as I set it originally to 600GB and then reduced it to 300GB.

I tried the manual and found pictures of the log file but not how to determine why I am having problems.

Any help would be much appreciated.

Thanks.