Try looking for existing shared projects in your solution with broken/missing files and an unexpected folder structure.

When I encountered this, I found a shared project in my solution with folder names that appeared to be GUIDs, and within each folder, a file that I previously created. Those files showed as broken/missing though.

I'm not exactly sure how that happened, but once I removed that shared project, I no longer saw that "Multiple shared folders are not supported" error and could add shared project references again.