Question : I noticed at first my password to access OpenX didn’t work any longer. I reset it with the main password reset tool.

Now, I still can’t log into /account-settings-banner-delivery.php under the admin account. Asks for a password but nothing works.

I deleted prepend and append fields out of the banners table. The users table had no other users besides the admin in it. Also; in the banners table I noticed that <htmltemplate> has malicious <iframe>

Ans : Yes, you can remove the htmltemplate values from tables, this will not cause system. Just make sure, shouldn't remove for your HTML banners, because that will place the content on this field.

And also please visit (here) and check with your system.