I tried installing and uninstalling more than 10 times. Including rebooting. The admin directory is never there. So no luck there.
I use a new (mantis) admin username and the MySQL root password. It installs without any errors, but still no admin directory. All other mantis directories exists, including the MySQL tables.
At first I tried to install the 32bit run version but that failed with no message (just got the prompt back). Is it possible that this broke something?
It is probably not too difficult to install mantis manually so if there is no solution for this I will try that approach.