I didnt log the error messages but when rebasing Fedora Silverblue, there was an issue that I encountered:

In Silverblue 34 I had at some point clicked "enable workstation repositories" in Gnome Software and forgotten about it. This added a layer with the relevant rpms.

As this rpm is now in the main compose, rebasing streams caused problems. I had to reset the Fedora 34 install to not include the layered packages before rebasing.

As the prompt was from the GUI, I suspect a lot of Silverblue users will hit this.

other than that it was a smooth upgrade.