abacab
So, Windows Defender runs in the background when you use a 3rd party AV on Windows 10, but in my case, since I had uninstalled my 3rd party AV it was running in full real-time scan mode.  I had to go into Windows Group Policy editor to disable Defender.  That is only available in Windows Pro, but there are registry edits available for Home users. https://www.tenforums.com/tutorials/5918-turn-off-windows-defender-windows-10-a.html
 
Once I disabled Defender, my installer connected just fine.  I had tried nearly everything else including disabling firewalls, etc.  It really surprised me that Defender was interfering with an internet connection, but I'm now convinced that Microsoft has turned Defender into a super net nanny!

I opened defended and turned off the real-time and cloud-based protection. Are you saying that isn't enough? You have to go into the Group Policy Editor?

Anderton
 
I opened defended and turned off the real-time and cloud-based protection. Are you saying that isn't enough? You have to go into the Group Policy Editor?

So I assume turning off protection did not help.
 
Not sure what Defender component was blocking my installer, but completely disabling Defender so that even the service was no longer running is what freed up my installer's outgoing connection.  This was the last thing I tried after everything else.  Makes no sense to me!
 
Microsoft has made it very difficult to completely stop Defender on the latest Windows. I think that Microsoft is trying to protect us from ourselves in a very dumbed down way.  This is probably a good thing for some folks, but not us! 
 
Even if you install a 3rd party AV, Defender continues to run in the background.  It just deactivates Defender real-time protection, but continues to perform scheduled scans.
 
Group Policy Editor lets you act as administrator to completely disable it. 
 
I'm not saying this is what is causing your issue, but based on some uncanny similarities in our problems, it is probably worth a shot to check out. I wonder if Microsoft changed something in a recent update that is messing with the way certain developers coded their apps to download software, that appears to be behaving like malware.  A stealth false positive, maybe?
 
If that's not it, I wish you luck!

Craig perhaps you have done this already but have you  turned off the Firewall and all virus software?
Other things to do
If it's still not working (and you're on a Windows computer) then it is possible that you are running your DAW in compatibility mode, which interferes with the authorization. Right-click on the DAW icon -> Properties -> Compatibility tab. Then deactivate Compatibility mode if activated and try again. Do the same for the Online Installer.

For all the trouble you have been having I think you should format the drive and install fresh or at least try a now hard drive and reinstall. it would have taken less time them solving this problem and others you have posted here.

peace!   

I'm having problems only with XLN's Online Installer program. It's not worth taking the time to re-format the drive if I can't be certain that will fix the problem. Besides, XLN gave me a test file to download from their servers and it worked without problems, so it's not my internet or ISP. it's only the Online Installer that refuses to connect.
 
Fitzj, I've tried running the online installer in every compatibility option known to man. XLN told me to remove all the AD2 Dlls so they're not a factor. 

Have you tried removing everything XLN, nuking all XLN related folders (Program Files, Program Files (x86), Program Data, Documents, Users\YOU\AppData\Roaming) you could just rename them if allowed, or cut/copy/paste so as to be able to return to your current state and not loose any personalized stuff, nuke all references from the registry, rebooting and starting from scratch?
 
And how about on another computer (same location), can you get it to hook up?

Matron Landslide
Have you tried removing everything XLN, nuking all XLN related folders (Program Files, Program Files (x86), Program Data, Documents, Users\YOU\AppData\Roaming) you could just rename them if allowed, or cut/copy/paste so as to be able to return to your current state and not loose any personalized stuff, nuke all references from the registry, rebooting and starting from scratch?

 
That is probably the "nuclear option," but I'm not sure how those things could cause the Online Installer not to connect to the XLN servers...maybe the error message isn't really describing the nature of the problem, though. 
 

And how about on another computer (same location), can you get it to hook up?

I have a different computer, same location, and it also won't connect but has a completely different problem where it gets into this loop where it updates, restarts, says there's a problem with the installation and needs to update, updates, restarts, says there's a problem with the installation and needs to update, updates, restarts...rinse lather repeat. Maybe that indicates what you're suggesting about nuking everything would indeed help.

I was also thinking (it sometimes happens), that from memory I think the XLN stuff use to required re authorization after major Windows 10 updates (I am not 100% certain now, because I have had updates disabled for a long time), could be coincidental, but it couldn't hurt to check your Windows 10 update history to make sure there aren't any failed updates or such, a long shot, but you never know, computers and software can be funny things, easy to check anyway.
 
2 different computers from the same location, I would be looking (rightly or wrongly) at settings in your network (router firewall or whatever) that for whatever reason became a little skewed towards XLN's system, probably very unlikely, but again anything is possible and an easy thing to check, you have an odd problem.

Anderton
I'm having problems only with XLN's Online Installer program. It's not worth taking the time to re-format the drive if I can't be certain that will fix the problem. Besides, XLN gave me a test file to download from their servers and it worked without problems, so it's not my internet or ISP. it's only the Online Installer that refuses to connect.
 
Fitzj, I've tried running the online installer in every compatibility option known to man. XLN told me to remove all the AD2 Dlls so they're not a factor. 

I get that, but you have had two other problem that you have posed here in the last few months. :) (warning signs)
You can't just keep patching things up. It's a refreshing feeling to clean house. You are most likely running ragged as always, over doing your work load... lol But in the time that you have spent on this issues, that should have not been an issues... You could have reinstalled your system 3 times. I know, I just did it on one of my systems. :) But that's just me. lol
If I see you have one more computer issues, I'm going to buy you a hard dive myself! lol
 
peace! :)
 
 
 

As an old IT guy, this sounds like a DNS issue to me. Perhaps your computer has an old DNS record in the cache and is trying to resolve to an obsolete IP address for the server. To clear the cache, open a command prompt as Administrator and type:

ipconfig /flushdns