The weekly leaderboard is offline. It tells me it is offline, and has for a couple of days.

I have the same problem. I purchased the game only 3 days ago, so I have installed it with the latest setup file (setup_infested_planet_2.1.0.2.exe). Also purchased and installed the DLC (setup_infested_planet_tricksters_arsenal_2.1.0.2.exe) and everything else is fine.

On the left bottom of the "Weekly Challenge" screen is the message "Online Leaderboards not available".

I had the 2.1.0.15.dmg file the first time I posted. And, it was never online.
I just updated to the newest, and still offline.

I do not yet have the add on DLC.

What is the actual site that this connects to?

After installing the Trickster's Aresenal addon.

Game files have been altered.
Leaderboards have been disabled.

Bad news.

I've got this answer from Alex Vostrov, the developer of the game:

"Thanks for buying the game.

Unfortunately, the only version that supports online leaderboards is Steam. Online leaderboards is something that Valve provides to developers. I wouldn't be able to do them, otherwise. The best I could do for the DRM-free version is a high score."

Thank you for your followup.

It is a fun game. Hopefully they will make more missions.