After checking twice for upgrades on a fresh 25.7 install (see https://forum.opnsense.org/index.php?topic=49932.0), the process started as usual.
After some time I got a red popup indicating:
Danger
Unexpected error, check log for details
Don't reload, reboot or otherwise do anything here - just wqait and monitor the log indicating progress. It took me a couple of reinstalls and leaving this alone by chance once to realize after more than 10 minutes the upgrade had resumed and finished.
The red error popup happened around this time in the upgrade:
[20/97] Extracting nss-3.118.1: .......... done
[21/97] Upgrading zstd from 1.5.7 to 1.5.7_1...
[21/97] Extracting zstd-1.5.7_1: .......... done
[22/97] Upgrading boost-libs from 1.88.0_1 to 1.89.0_1...
[22/97] Extracting boost-libs-1.89.0_1:
Well, I tried to do that (check logs) but then accessing the web UI showed this instead:
403 Forbidden
After reinstalling and reaching the same stage in the upgrade, I noticed it resumed and kept going in the background (the log continued showing progress after a long time, at least 10 minutes).
I had enabled SSH access and connected during this process, everything looked normal with a busy pkg-static process indicating the upgrade continued.
Eventually as noted above the UI came back with the reboot message:
The upgrade has finished and the system is being rebooted at the moment, please wait...
In the background the log indicated a completed upgrade.
Upon reboot after a few more minutes the UI came back again prompting for login.
So, other than the worrying red error message, this time around the upgrade required much more time than usual, waiting it out was unusual but required.
Plan for this downtime next time you upgrade. :)
I was lucky this particular router has an easily accessible serial interface so I could monitor and check boot sequence. I have 2 other devices I accidentally bricked because I didn't wait, but they will require HDMI + keyboard to be fully reset.