Sorry, I thought it was not of importance, because both, trying to run it as normal user or as su produces the same error message.
Now again as su, because the files are owned by root:
debian:~# cd /usr/lib/AntiVir/
debian:/usr/lib/AntiVir# file antivir
antivir: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.2.5, dynamically linked (uses shared libs), stripped
debian:/usr/lib/AntiVir# strace antivir
strace: exec: No such file or directory
execve("/usr/bin/antivir", ["antivir"], [/* 15 vars */]) = 0
Hm, that sounds, as if there could be indeed a problem with 32bit programm in 64-bit debian, isn't it? But initially it worked very well without explicitly changing into the chroot. But thats a hint I will follow soon.
Thank you very much for your kind help, uselpa. I'll post, if I get it working again. thanks again.