I have found that when you enter a hyperlink into Excel Online, open in the Excel Rich Client, Save, and then re-open in Excel Online, the hyperlinks are update and subsequently broken. This is because, in the Excel Rich Client all hyperlinks and paths to all supporting files are automatically updated before you save or publish an Excel workbook as a Web Page to ensure that all the links are up-to-date at the point of Save.
So if you enter the hyperlink (in Excel Online): https://contoso.sharepoint.com/Documents/Source.xlsx?d=<guid> it will be changed to a relative link: Source.xlsx?d=<guid> and you will see the below error:
There are two ways to fix this.
1). Use the =HYPERLINK() function.
2). Disable the updating of these links in the Excel Rich Client, by going to Excel Options > Advanced > General > Web Options > File names and locations > Update links on save.
I have brought this up to our product group.