A little more information. It looks like Calibre Companion might be sending a path that isn't strictly compatible with Android 11.
This is the path to the books in CC and the error message I get from Fbreader.
It looks like CC is using a base path of "/external" instead of "/storage/emulated/0"
For now, I am using Librera which is working, but I would love to go back to Moon+Reader