Remote Console not loading via http after upgrading to v3.51

Hi,
I upgraded two Venus OS installations.
One on Raspberry Pi 2 worked fine.
The other on Raspberry Pi 4 (from v3.31 to v3.51) is still responding via http, but the remote console (v1 as well as v2) stops loading:


The remote consoles are only loading via https, although Network Security Profile is set to Unsecured.

Bye-the-way: is it possible to revert to the backup-firmware without the need of connecting a display+keyboard to the Pi, e…g. by following a specific power off/on sequence?

Bye, Alex