I always download every of my new versions once, after upload. I also just re-downloaded 0.18.3 now to try reproducing your problem. Both downloaded ZIP files work in my very late game with all items present. So no problems on the file itself, at least from my zone.
But yes, there were times that I could not access this mod portal as well. (Not using in-game mods manager so I cannot tell.) I could not download or read discussion posts here. But those cases were rare that I only met that situation twice in the last 12 months. But that usually did not persist for more than a few hours.
BTW, you can try the followings:
1/ Download another mod. See if the problem only exist for this specific mod, or actually any mods.
2/ If your mod list is showing the current mod version is 0.17.18 / 0.18.3, you may need to delete the file first before retrying download. I am not sure how the game handles corrupted file attempts, but removing the file ensuring the new download will start clean.
Let me know if the problem persists. And you can provide more info (like screenshots) to describe more details.
