Hi Alana,
I tried to reproduce this issue using Chrome from my side. However, I can upload attachments fine both in Office 365 pre-upgrade and after-upgrade. Are you using Office 365 pre-upgrade or after-upgrade?
Actually, the uploading attachments mechanism of Office 365 may be not the same with Hotmail. In Chrome, there can be a compatibility issue with this feature. I suggest you contact the support from Chrome to check if there are any settings that can enable this feature. Meanwhile, officially, we suggest you use Internet Explorer to ensure you have a better experience.
About this issue “it won't even let me login to my 365 account”, which browser are you using when encountering this error? Please check whether you can login to Office 365 well with Internet Explorer. If the issue persists, I would appreciate it if you could provide related error messages and screenshots for troubleshooting.
Thanks,
Kevin Cui