Hi SpicySoftTofuSoup,
Please check the following to troubleshoot this issue:
1. Clear your Office file cache,please refer to the article that Microsoft Excel cannot access the file on SharePoint:
https://gcits.com/knowledge-base/microsoft-excel-cannot-access-file-sharepoint/
2. In Excel, select Options >> select Trust Center in the left-hand bar, and then click “Trust Center Settings..” Button >> go to the following sections separately: Trusted Locations, Trusted Documents and External Content, and then select "Allow....to
be trusted" Checkbox
3. Delete the file and upload again
Here is a similar issue for your reference:
https://social.technet.microsoft.com/Forums/en-US/d288eab1-91cf-4406-a940-d1d94b79e767/were-having-trouble-connecting-to-the-server
"SharePoint" forums will be migrating to a new home on Microsoft Q&A !
We invite you to post new questions in the "SharePoint" forums' new home on Microsoft Q&A !
Regards,
Echo Du