Error on updating ticket in default queue

hello everyone
I have a RT 4.4.1 and I have a default queue named RT-Test. Someone sent a ticket to this queue. as this queue administrator, I want to resolve this ticket. the problem is when I want to resolve (or any change status) or reply this ticket, RT denies to do so and gives me this Error:

The system is unable to sign outgoing email messages. This usually indicates that the passphrase was mis-set, or that GPG Agent is down. Please alert your system administrator immediately. The problem addresses are:

I searched on the internet for any clue. someone said check that your GnuPG is not enabled in I do so and there was nothing.

can anybody help me with this?

many thanks

Is GPG-Agent installed, and if so, can you un-install?

Removing GPG from the scenario, my guess would be that you have a problem with certificates.