[Tails-dev] error 7424

I tried to test the incremental upgrades today but in the end the only
thing I managed to do was to hit the same error over and over again:

https://pad.riseup.net/p/error-7424

I tried with and without root password. I reinstalled Tails 0.15 twice.
The USB drive used to be a Tails 0.16 and I used "Upgrade from ISO" with
Tails Installer from a Tails 0.16 to install Tails 0.15 on it.