The only error windows I get come from the stuff that is added by the server, where some nonexistent objects are accessed.

I did not see a single Applet warning window, I also did not see any Applets.
Ok, I did not look at all pages (btw, some nice pictures!), but they seemed all quite similar to me (not the content, but the structure).

Applets are small programs written in Java (not to be confused with Javascript). If these small applets open new windows, some browser put "Warning: Applet window" somewhere on the window.
This is because applets can (under certain restrictions) access your system (but only if they are signed, and you have to confirm that you allow a signed applet to run).

My guess is that there is a bug in your Page Builder program...