Interesting, thought I would try the XLN user forum to see if others were were having a problem. 
 
https://www.xlnaudio.com/forum/viewtopic.php
 

WE HAVE SHUT DOWN THE FORUM

 
I might start a new thread as to not detract from this one which is a different issue .. maybe not

Did this issue ever get resolved?  I was searching for a different XLN error and ran across this thread on Google...

abacab
Better plugin installers, and plugins that don't ever crash DAWs, would be a good start...

That was the beauty of Reason before VST support.

abacab
Did this issue ever get resolved?  I was searching for a different XLN error and ran across this thread on Google...

Nope...which is problematic, I have older projects that used AD2. I pinged XLN before going to NAMM, and they told me they didn't have an answer but they'd let me know if they did. At this point I'm not holding out much hope. 

Well that sucks...

The symptoms sound like a security issue, i.e. either permissions (so needs to be run as Administrator), or a TLS/SSL/Certificate issue.
 
I would try the following:
 
1. Install all the latest Microsoft security updates to ensure you've got the latest SSL cyphers / protocols.
2. I'm not sure if the XLN installer uses flash, but updating to the latest Adobe Flash player might help for the same reason as 1.
3. Just in case you're somehow hitting an old server, you could flush your dns cache by firing up a command prompt and typing ipconfig /flushdns 
4. Check you've not got any strange entries in c:\windows\system32\drivers\etc\hosts that could be redirecting traffic (doubtful, but no harm in checking)
5. Disable all firewalls (both on your PC and router)
6. Run as administrator
 
M.

 

I found the problem! When I dug deep enough into Firefox's security warning, it said there was a certificate it did not recognize. I opened Manage Computer Certificates, deleted the certificate, and the XLN online installer worked.
 
If I hadn't tried to open the installer with Firefox, I never would have found the problem. I'm not surprised XLN support couldn't find it...this was definitely a corner case.
 
I have no idea why this didn't trip up any other installers.
 
Craig 

Thanks for reporting back. Glad to hear you got it working again.

Great news Craig,
 
I am glad the you figured out the problem with AD2.  You might want to share you findings with XLN so they \have your findings in their arsenal as this problem may rear its' ugly head elsewhere in the future.

Jesse G
Great news Craig,
 
I am glad the you figured out the problem with AD2.  You might want to share you findings with XLN so they \have your findings in their arsenal as this problem may rear its' ugly head elsewhere in the future.

Agreed! I already let them know