During the upgrade or frequently in the logs or simply when starting ldap-tools you notice an error meessage:
error : Unknown IO error
There is a bug for it. You should ignore this message as it appears to be only cosmetic.
Or if this troubles the execution of a joinscript, you may try this solution. (Adjust the joinscript!)
unset http_proxy HTTP_PROXY ftp_proxy && univention-run-join-scripts --force --run-scripts 92univention-management-console-web-server.inst