Hi Samuel,
Thank you for being so patient with me...
This morning I have started working on the problem again and I am pleased to say that I have made *some* progress... But I am still basically clueless about *what* goes wrong!
First, let me mention that I have *no* problem using *your* site (i.e., *this* support forum). Hmm...
Next, when I try to log in on my own machine, I do NOT get a popup. Instead, I get a runtime error alert: Line 96, Object expected (again, this does *not* happen on *this* site --mental note to self: check if JS settings between Internet zone and Local intranet zone differ?!?).
When I click on No for debugging, I get the login screen in my main (and only) window. admin/admin --> Login Succesful!, followed by an alert: " error.location is null or not an object" --sure, since I did not get the original popup.
I hit my Back button to get to the forum, and see that I am logged in as " admin" , and the admin control panel button shows.
I click that button and get an alert: Line 213, Object expected; I click No for debugging.
At this point, I see the *left* frame of the admin screen (i.e., the menu) in my main/only browser window (should this be a popup as well?)
The *right* frame tells me that " Too many users are connected" (not true, I' m the only one using my PC, but this may be a misleading message --the error may be elsewhere).
When I click on an option in the menu, say: Basic Configuration, the corresponding screen appears in the right frame.
So, summarizing, there *is* progress, but I' m not quite there yet!
By the way, my remark:
However, in order to get good performance, you first have to get it to work, correct?
was *not* intended as criticism of your software (which clearly works *very* well for many people!), but as a " tongue-in-cheek" comment to myself (I hope that did come across correctly --English is not my mother tongue and it is usually best to avoid irony when you are very tired...

).
Anyway, thanks again for your help so far and I will keep you posted if I make progress. If you have any ideas based on my description of the problem above I' d love to hear from you!
Marcel