Hi gstyl,
According to your description, I suggest you follow these steps below to narrow down the issue:
1. Try to edit the excel file with other browsers.
2. Try to edit the excel file with the other system, such as Windows 7.
In addition, I would like to collect some detailed information:
1. If there is the same issue when you open other documents.
2. Could you open or edit the excel file in browser?
If the issue persists, you can provide the image for us with this method:
-Click “Use rich formatting”, and then refer to the screenshot below to upload the image. 
Thanks,
Eben Liu