Alright, now I think I remember. I got this error when blockland would attempt to download latestmanifest.php but instead would receive a different file due to a redirect.
What it means? no clue.I fixed it by disconnecting my internet, boot Blockland, close it, and reconnect the internet.
It could be cloudflare trying to insert an interstitial page that the launcher can't handle.
No, Blockland is protected by Cloudflare. It's a totally valid point.
The whole blockland.us domain is protected by cloudflare, you'll see their name servers if you look it up on whois.cloud.blockland.us is only for static files anyways, the launcher tries to load http://update.blockland.us/latestVersion.php.
Ok I tried a few things and it worked, thanks for the help guys :)