Eudora Client caused Plugin Error

Hi experts!
From Eudora client I found a PGP encrypted message.
When I opened it I got the following error message:
Plugin Error: %3C0880.0001%3E
"plugin no longer available"
May you please help me know what the problem is? I checked almost all PGP stuff and everything is running fine.
Also, I disabled email checker in AVG but cannot – it displays "active" all moment
I am using Windows XP Pro operating system and I use:
AVG Free v.9.0.819
Virus DB: 271.1.1/2891
Internet ADSL
email server is different from my ISP
Eudora email client
PGP 6.5.8
Zone Alarm
