Hello Micha, thanks for your prompt response and sorry for bothering you.
The first command pointed me to the actual root cause: aqhbci-tool4 was taken from /usr/local/bin and was built from source by me last year on a Debian 8 system. I did not consider this location because I have created the Debian 9 system essentially as a new install. But presumably I have also fetched this binary erroneously by taking over some personal scripts at this location from the previous system. Kind regards, Peter
