The upload directory is the one that points to jdownloads directory. So if you have the same structure then it would have ai.delta-software.com where previously it had delta-software.comNormally it is the same as the path to the temp folder in Joomla! Global config - server settings but with jdownloads rather than tmp.
But having looked at your new site with Firefox I do not think the root problem is with jD. I went direct to another pagehttps://ai.delta-software.com/unser-angebot/produkte/amelio-logic-discovery.html#tab1
and got the warning below
"Firefox detected a potential security threat and did not continue to ai.delta-software.com. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details."
When I went to the link you gave Firefox warned that a web page was slowing the site down, this was before I tried any Download