Security Problems with my site/Popup will appear

[barcena]

Hi guys I do have another site that apparently is having security problems, allow me to explain:
Since this morning every time you try to open a new threat or reply a popup will appear and ask you for a login/password, when you cancel that thing a page will promp with this message:

401 Error - Unauthorized
HTTP Error 401.5 - Unauthorized: Authorization failed by an ISAPI/CGI application.

I haven't done anything on my server CP (I never touch that) and I don't think there's any option I could have screwed on the CP that will lead to that error, I'm pretty familiar with vbulletin Admin CP and I have not done anything weird or out of the ordinary, probably a couple of banned users and not much else.

The popup is the kind that appears in those security sites when the admin will allow permissions to only certain people to see his work, lets said, and mine appears inside the forum you being logged in as an Admin, go figure!. I haven't done anything that can lead to that, last night it was perfect, this morning that thing started to show up. I dont have to say that nobody can post or do anything on the site.

Any ideas?.

I will much appreciated your help bc the site is close and I'm getting nuts.