I had what seems to be the same problem while upgrading from 25.7.5: error popup on the update screen while installing the upgrade, 404 when I reloaded the update page, 403 from the root URL (which should've given me either the dashboard or the login page). But it came back after a few minutes.
Since earlier posts here have expressed concern about whether the core package might be broken by this upgrade, I downloaded a config backup and the latest installer as a precaution, then rebooted my router as a test. It started up successfully and seems to be working as usual.
Since earlier posts here have expressed concern about whether the core package might be broken by this upgrade, I downloaded a config backup and the latest installer as a precaution, then rebooted my router as a test. It started up successfully and seems to be working as usual.
"