The installation on my Fedora 40 machines seemed to install fine, but ran into a number of issues on the 3 machines.
Two have Nvidia cards, but both of them reported no usuable GPU after installation, and reboot resulted in same.
So, making update useless since more work done with Nvidia card than CPU cores.
Also, it installed in /usr/local/bin rather than the previous location from Fedora BOINC.
One machine didn't have that in path, so had to modify startup script for client and manager.
Then sometimes starting the manager would get a message about invalid RPC (I think)..
I just removed the boinc-client and boinc-manager, and then removed the new repo from /etc/yum.repo.d and then reinstall them
from default fedora repos going back to the regular, and again the GPUs were seen, and no errors.

Note sure why install instructions mention yum?
sudo dnf install dnf-plugins-core
sudo dnf config-manager --add-repo https://boinc.berkeley.edu/dl/linux/alpha/fc40
sudo dnf config-manager --set-enabled boinc.berkeley.edu_dl_linux_alpha_fc40
sudo rpm --import https://boinc.berkeley.edu/dl/linux/alpha/fc40/boinc.gpg
sudo yum install boinc-client boinc-manager

Currently 7 timezones away from two of machines, so didn't have time to really work on it.
Perhaps try again later, or if there is something specific to check.
Thanks.

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First thought - incorrect drivers installed. Which Nvidia GPUs are you using? Which drivers (source and version) are you using?