Same issue here, running HAOS on RasPi. After an automatic update to 2026.1 the UI does not load, and core check gives the “Object of type DockerMount is not JSON serializable” error. The observer page says everything’s fine.
I’ve now downgraded the core to version 2025.12.4 that worked for @troyc, to no avail. Still the same error and no GUI.
I wonder if the backups are of any use as they cannot be restored. The backup restore command says the backup is partial. After I had to make a clean install from scracth a month ago due to an update breaking everything and backups being only partial, I think I paid extra attention to take full backups this time. Apparently that didn’t work out.
As a sidenote, the console is flooded with errors related to Bluetooth (at least “Cannot set baudrate”), and occasional warnings of undervoltage. These were not present after I had done the fresh install in December.
Edit: Did a clean install and then managed to restore a backup (taken Jan 4th) right on the welcome page. The installed version is now 2025.12.3 which seems to work. The GUI loads ok, but core check still gives the “Object of type DockerMount is not JSON serializable” error! Strange. The Bluetooth errors and undervoltage warnings are still there as well.
Might the “Not serializable” error and GUI not loading be of different origin?