Forbidden (403) message when using API

I'm getting a 403 (or 400) "forbidden" message when I try to access my API:

Client error '400 Bad Request' for url 'https://api.zotero.org/users/[XXXXX]/items?locale=en-US&itemType={'annotation'}&since=0&format=json&limit=100'

I can provide my user number instead of XXXXX if helpful. But I wondered if this was a broader problem? Or if I've been blocked etc.

Thanks
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