Ok, I found it. The logs showed me that everything ended successfully. I tried uninstalling the agent using the management platform, which actually does a better job than aexagentutil /clean (this still leaves some files in place). I then tried pushing it again, without success, although the log showed otherwise. But in the log the path was mentioned as "E:\program files\Altiris\" instead of "E:\program files\Altiris\altiris agent". So I tried the manual install again with that path and it worked.