What does the “start page” mean, would you please take a screenshot to me? It could help me further analyze this issue, thanks. Have you ever downloaded the files to local computer from SharePoint site and checked if Office 2016 for Mac is able to open them OK? Have you login with your account in Office for Mac? Such as visit the SharePoint site via browsers and choose the files open by local Office applications, or open the local Office applications first and browser the location or URL of files Please let me know how you open the files in SharePoint site.
I need more information to analyze for this issue.
