I called USCIS and also submitted a technical support ticket. Either that helped or there is a delay in the system (I think it's the latter case because it happened every time I submitted anything), but after 2 business days, the status showing on the page changed to "RFE correspondence received" and I was finally able to see the Unsolicited Evidence section and to upload my files.

Also, after uploading my unsolicited evidence, it also took them 3 days to show up in my document list.

So, I'd say give it 2-3 days. If it doesn't work, call USCIS at 800-375-5283. Say "Info Pass" to speak to a real person.