I solved the problem myself and am sharing it with others.
After the update, the system wouldn’t boot on kernel 6.12.54 (Qubesos v.4.2.4). At startup I selected kernel 6.12.47, and the system booted, but the cubes and templates didn’t load.
I manually changed the kernel for each cube in their settings: in the “Advanced” tab I switched from 6.12.54 to 6.12.47. The cubes and templates then loaded.