Zxchat cannot be actived

I have activated the Zxchat for some users. For few of them work fine. But some users are prompted an alert while they login zimbra
"Zxchat zimlet is disabled for delegated access or resource accounts". On the lower right hand corner, the Zxchat icon shown in red. I wish to see to can I solve the problem.

Thanks a lot.

Regards,
ELam