The delegated administrator cannot set an email signature from the user record.

How to grant a user delegated administrator permissions to set email signatures from the user profile; I have tried the following:

Allow editing the user's plain-text signature
# su - zextras -c "carbonio prov grantRight domain domain_name usr user@domain set.account.zimbraPrefMailSignature"

Allow editing the user's rich-text signature
# su - zextras -c "carbonio prov grantRight domain domain_name usr user@domain set.account.zimbraPrefMailSignatureHTML"

However, it doesn't allow saving the signature or viewing the created signatures in the list; clicking the "Accept" button does nothing—it's as if the system were frozen. Please help me with this.