I don't thinks it's a json problem because I triple check that… On another project it is working perfectly. And it is also good with @Gargron #ActivityPub tuto (https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/)…
Why is this step necessary? mastodon.xyz might not be the server where you are registered, so we need to redirect you to your home server first.
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