I can reproduce this with arxiv.org. Regression in 5.0.111.
inject.js:478 Uncaught (in promise) TypeError: Failed to fetch
at Object.unpackString (utilities.js:92:27)
at Object.saveSingleFile (chrome-extension://kamnafiakhhkladalejcocpigcjbiidm/connector.js:248:59)
at Object.receiveMessage (chrome-extension://kamnafiakhhkladalejcocpigcjbiidm/messaging.js:83:20)
https://forums.zotero.org/discussion/comment/438446/#Comment_438446
I can reproduce this with arxiv.org. Regression in 5.0.111.