Thanks for your response however re-upgraded was not an option for me.
I had too much trouble already with the upgrade. Besides that I've modified some files. E.g. isp_username.plugin.php as I don't use the web prefix, but use the domainname.com_admin structure and isp_username.plugin.php to disallow to edit the e-mail. This allows users to login with their normal e-mail account (firstname.lastname@example.org). For dovecot I've added auth_username_format = "%Ld_%Ln" to allow also normal e-mail login. I've increased the user_username column of isp_isp_user to 50 and my linux fedora 12 allows the large user names (length of 38is my largest one).
Anyway, I manager to fix the loss of the Administrator checkbox. I first made a backup of the database. Used the form editor to changed the isp ISP user form and editted the current user_admin and renamed to user_admin2. Then I made a new user_admin checkbox with a name and defaults. Then I run an sql update:
update isp_isp_user set user_admin = user_admin2
this will restore the admin flags.
Now I'm able to see the administrator checkbox when I'm editing the e-mail details for a customer. So far it's working properly now!
So thanks to your direction I was managing to fix it.