APT 0.6 sometimes fails with gpgv

[Ognyan Kulev <ogi@fmi.uni-sofia.bg>]

Hi,

I'm using apt 0.6.25 on sid.  When after some days with apt-get update I do 
apt-get update, the following message often occurs:

Get:1 http://ftp.uni-sofia.bg unstable Release.gpg [197B]
Get:2 http://ftp.uni-sofia.bg unstable Release [34,1kB]
Hit ftp://ftp.uni-sofia.bg ../project/experimental Release
Hit ftp://ftp.uni-sofia.bg ../project/experimental/main Packages
Hit ftp://ftp.uni-sofia.bg ../project/experimental/contrib Packages
Hit ftp://ftp.uni-sofia.bg ../project/experimental/non-free Packages
Hit http://ftp.uni-sofia.bg unstable/main Packages
Hit http://ftp.uni-sofia.bg unstable/non-free Packages
Hit http://ftp.uni-sofia.bg unstable/contrib Packages
Fetched 197B in 0s (693B/s)
Reading Package Lists... Done
W: GPG error: http://ftp.uni-sofia.bg unstable Release: Unknown error executing gpgv
W: You may want to run apt-get update to correct these problems

All following apt-get update in the same day don't show this message.

Is this a known problem and what's the explanation?

Regards,
ogi