If you use the avast! for Mac product, it will prevent the Mac Client from contacting patch.lotro.com and updating.
The end result is the "classic" error: "An Update Error Occurred."
In the System Preferences control panel for avast!, "Web Shield" is the culpret.
If you have it enabled, the Mac Client will not get started.
You can either "disable" Web shield completely, or select "Advanced" and add "patch.lotro.com" (HTTP) to the list of exclusions.
(For the Bullroarer client also add "moria.patch.lotro.com")
I assume that this issue will also occur with other anti-virus programs which scan web content.
If you are having the "An Update Error Occurred" problem, try disabling your anti-virus scanner. If it works, please report your results back to this thread!