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It seems unlikely they would be intermittently blocking addresses.
However as Vodafone's blocking is DNS based, the easiest way to stop it is to not use Vodafone's DNS.
Change your DNS setting in the router to someone else. I like to use Cloudflare and Google. 126.96.36.199 and 188.8.131.52.
The ifit-wolf.s3-cdn.ifit.com site is badly coded/setup there are problems accessing the specific hosts. From what I can see that site fails from other major UK ISPs too!