spent a lot of time figuring out how to cure this. During setup I chose the DSA certificate options, gave the proper local name (domainname.com format without www.)
and setup concluded successfully; however port 81 gave no response.
Cure: modify //root/ispconfig/httpd/conf/httpd.conf such that the line
SSLCertificateFile points at .../ssl.crt/snakeoil-dsa.crt and
SSLCertificateKeyFile points at .../ssl.key/snakeoil-dsa.key and
SSLCertificateChainFile points at .../ssl.crt/snakeoil-ca-dsa.crt
Then restart ispconfig: service ispconfig_server restart
Apparently the setup script does not perform these adaptations automatically and/or properly in reaction to the DSA encryption choice.