A couple months ago, I visited this website and downloaded some free CAD blocks. Ever since, every time I open AutoCAD, the CADforum website will open along with it. I don't know how they became connected and I don't know how to disconnect. I have downloaded blocks in the past and this has never happened before. My shortcut only contains switches to suppress the splash screen and load my preferred profile.

This is a notification about a new version of one of the LISP add-ons you are using. You can either update it or suppress the checks. See:https://www.cadforum.cz/en/how-can-i-find-out-which-version-of-lisp-freeware-is-loaded-tip14399

I don't have any Arkance lisp routines loaded. I only downloaded some blocks for lavatory fixtures.