Experiencing the same. Seems like the arXiv API is currently just very slow. Pasted the failed URL into the browser's URL bar and got a response, after a long time of waiting for the API (that is longer than the timeout of 15000ms set in Zotero Connector).
I also have this same issue, where saving with from arxiv gives: "An error occurred saving with arXiv.org. Attempting to save using Embedded Metadata instead." I am using Chrome 120.0.6099.199, zotero connect 5.0.114, and zotero 6.0.30 on Windows 10.
Upgrade Storage
==Chinese (Google Translate) ==
抱歉，谷歌翻译文本：
经历一样。 看来 arXiv API 目前非常慢。 将失败的 URL 粘贴到浏览器的 URL 栏中，并在等待 API 很长一段时间（比 Zotero Connector 中设置的超时 15000ms 长）后得到响应。
This has also broken my arxiv RSS feeds.
==Chinese (Google Translate) ==
抱歉，谷歌翻译文本：
我也有同样的问题，使用 arXiv 保存时给出：“使用 arXiv.org 保存时发生错误。尝试使用嵌入式元数据进行保存。”我在 Windows 10 上使用 Chrome 120.0.6099.199、zotero connect 5.0.114 和 zotero 6.0.30。
这也破坏了我的 arxiv RSS 提要。