Catalog errors like this are usually down to a case sensitivity issue they usually show up during a duplication, restore or verify rather than during a backup
As you can see from the structure it all goes under catalog\2\ followed by the client name and then the policy name
If in a policy the name gets changed or the policy gets changed (same names but different case) then it can make things go wrong.
You need to take a look to see if you have both CLIENT and client first - if so the files from the upper case on need to be copied into the folder structure of the lower case one - then the upper case one deleted - this can eb tricky sometimes if you have the same named files in both - if so raise a support case
Once you just have one directory we add a softlink to the catalog\2\ directory
As yours is currently in UPPER case you would just do the following:
cd into the catalog/2 directory
ln -s client CLIENT
I have the feeling, as you say "if you put it in a policy on its own" that the case sensitivity issue may be at policy level rather than client level so what you may need to do it to go down one level deeper and add a softlink to the catalog folder
Now case sensitivity was supposed to be fixed in 7.5.0.6 which you get with 2.5.3 - so it shouldnt be happening to you!
What it may be is that the structure within the policy folder under catalog\2\ has become corrupt and so the client will only work when it is in a different policy - the answer may be to just create a new policy and put the clients in that so that it gets a new structure
As there is this possible corruption it would be worth logging a call to get it investigated further as they have a recoverCR tool that can be supplied to check and correct your de-dupe database which may sort it all out for you.
Hope some of this helps and makes sense!
