Following the instructions on https://fedoraproject.org/wiki/DNF_system_upgrade eventually I reached dnf system-upgrade download --refresh --releasever=25 which apparently completed without error, ending by directing to run dnf system-upgrade reboot. So, I did, and it booted normally. Nothing is running that isn't supposed to be running in a normal boot. dnf system-upgrade log reports no logs found. The only dnf string in journalctl -b is starting the makecache timer. This is not the first time I've tried those wiki page instructions exactly like this with same result. What's supposed to be happening that isn't?
It's supposed to boot into an offline updates mode, and do the upgrade. Please file a bug and attach logs ("journalctl -b -1" after you boot into standard desktop instead of offline updates mode). Thanks.