It appears CloudFlare no longer likes the way it is approached by MailerLite's RSS feed fetcher and throws up a challenge screen. I have tried a number of fixes and settings, and it doesn't work. However, if you proxy your feed and give that to MailerLite, it appears to work. Since MailerLight is a CloudFlare customer, they should be able to ask them to whitelist their fetch routines. If anyone has a fix, let me know.