(Please start new threads for new issues — the thread you posted to isn't related. I've moved this to a new thread.)

I am a bit annoyed that the DOI look-up is not working for weeks now

See Etiquette. There's no problem with DOI lookup in general. If you're having trouble with something, just report it without assuming that it's happening to anyone but you.

Can you provide a Debug ID for an attempt that fails?

(3)(+0000657): Translate: Could not find a result using DOI Content Negotiation -- trying next translator

(3)(+0000000): HTTP GET https://doi.org/10.[…] failed with status code 403

You're being blocked with a 403 Forbidden error by either doi.org or your proxy server.

Did you do many DOI lookups at once before this started?

Is your proxy server shared with others, such that all lookups from a single organization would come from the same IP?

We received one similar report the other day, I believe for the first time, so it's possible that doi.org changed to more aggressive blocking, but it might also be related to your proxy server.

Are you able to load a doi.org URL in your browser?

There was also a 403 for the doi.org request in the Airiti translator, which uses a normal GET request.

If you go to Tools → Developer → Run JavaScript in Zotero and run these, what do you get?

return (await Zotero.HTTP.request('GET', 'https://doi.org/10.1038%2Fngeo722', { headers: { Accept: "application/vnd.datacite.datacite+json, application/vnd.crossref.unixref+xml, application/vnd.citationstyles.csl+json" }})).status

return (await Zotero.HTTP.request('GET', 'https://doi.org/10.1038%2Fngeo722')).status

OK, can you run this and send the full output to support@zotero.org with a link to this thread?

return (await Zotero.HTTP.request('GET', 'https://doi.org/10.1038%2Fngeo722')).response

Great, thanks. It's from your organization's network security software:

Zscaler makes the internet safe for businesses by protecting their employees from malware, viruses, and other security threats

So, yes, you'll need to speak to your IT department.

I just deactivated the Zscaler. and the DOI look-up works...