Add item by identifier not working for DOI

I'm suddenly unable to automatically add items by DOI in both desktop and web versions of my library. I've confirmed the items I'm trying to add are correct DOIs. Also, I tested with DOIs I automatically added a few days ago, and Zotero cannot find those either. I can add items as usual via ISBN.

I get the erorr "Zotero could not find any identifiers in your input. Please verify your input and try again."

Example DOI that I was able to add via DOI lookup recently (4/23/20) but does not work today: 10.1525/aa.1971.73.6.02a00370
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