Saw this on the LOTRO forums. Will try it tonight and report here:

"Underlying Connection Was Closed Unable to Connect to the Remote Server"
The Underlying Connection Was Closed Unable to Connect to the Remote Server

If you are receiving this error currently one new cause of this issue that has cropped up is a trojan that has been setting Internet Explorer up to use a proxy address of http://127.0.0.1:50370 if you are getting the above error, double-check that you are not using that proxy setting.

Open IE (used by the game launcher for its system-wide connection settings) and click on 'Tools' > 'Internet Options' > Connections (tab) > Lan Settings (button) and disable the "use a proxy server" option and click ok twice.

Immediately after that update and then run a full system scan with your antivirus and antispyware tools just to be sure the infection is not active.