Besides the obvious that Great Plains 8.0 was never tested with Office 365, I've seen the following cause this type of error:
1. Remove the line - SmartlistEnhancedExcelExport=TRUE - from the Dex.ini file if it exists.
2. If you log on to the server or workstation as a local built-in administrator, then launch and login to Great Plains 8.0, can you then export to Excel?
3. Make sure there isn't any special characters in the SmartList data that you're attempting to export to Excel, as I've seen that cause issues.
4. Is it all SmartLists seeing this issue, including default.canned, or just custom SmartLists?
Check these and see if they help you. Maybe others on the communities are running Great Plains 8.0 and may have other information.
Thanks