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Hi Jim Naroski,
Thank you for posting your question in this community. From your description, you found that the ASPX file cannot be opened successfully in the SharePoint site which it connected with an Office 365 group.
According to this article about Types of files that cannot be added to a list or library, .aspx file can be uploaded successfully to a SharePoint document library, but you need to enable Custom Script of the site . And the custom script is blocked for the SharePoint site with Office 365 groups.
Hence, you need to enable the custom script of the site with Office 365 group by using PowerShell command to achieve it.
For your information:
To allow custom script on SharePoint sites with PowerShell command.
Best regards,
Dihao