if it's the same issue I've been experiencing, it's cloudflare's fault, they are forbidding connections from outside the browser, probably because of the ddos attack. Instead than downloading the feed, it downloads the cloudflare firewall page.
If you have programming skills and a homemade reader, you can solve it by downloading the feed using your mangadex credentials and a browser's user agent. Otherwise, just check from your Follows tab in the website. I assume it will be back to normal after the migration to business plan is completed.