I've been having this issues over the past several months. I have a Excel 365 workbook that lists documents (~750) stored out in SharePoint in numerous libraries. Within the workbook, I have the document name, other relevant information, and a hyperlink to the specific SharePoint list or library. I created the workbook on my desktop and subsequently stored it out in SharePoint. Not sure when or how, but one day, all of the hyperlinks failed, resulting in the error pop-up "The address of this site is not valid. Check the address and try again." Fortunately, I had a back-up so I did not need to reenter new URL's. Unfortunately, this has happened again, and I really need to figure out the cause. I've done some research and have done the following, but to no avail:

- made sure that "Update Links on Save" was not checked in Advance Options
- added the initial address for the SharePoint site to the Properties-Hyperlink Base. This resulted in a different error ("Unable to open"), did not include
then entire base address I included, and flipped the "/" to "\"

I also just discovered that the "corrupt" version of my workbook, which fails on my desktop, works successfully when I upload it to SharePoint.

Any ideas would be greatly appreciated, as I need this to work consistently regardless of its location.