There is a "dirty" quick fix if you need to login as admin later on.
Login to ISPConfig as admin and goto System -> Interface -> Main config
Remove the [CLIENTNAME] from the ftp user prefix field and save it.
Create a new ftp user named "admin" at any website.
Wait for the ftp user to be created and then reinsert the [CLIENTNAME] to the field at step 2 and save it.
Login with the new "admin" ftp user to ajaxplorer.