Cannot find COS for account

Hi people!
I'm doing a merge of 2 zimbra servers. (that are different versions, also compatible with the export module of zextras)

And for now, i'm doing the restore of the backup, to import in the last version of zimbra.
I do it, and 1/4 of the accounts in a specific domain, it appears this in the zextras external backup log:

Cannot find COS for account "acount@domain.com": falling back to original default

I dont know what does it mean "COS"...

What is so suspicious, is that this only happen with one domain (we have more than one), and it appears in 1/4 of the accounts in the same domain.

Does anyone know what can it be this?

thanks in advance!