In the hope of isolating the problem, I deleted the dashboard with the Vertical Stack card and restarted HA.
Now, the error is permanent on the stack-in-card card. Even if I navigate to another page, the error remains.
It sounds like the stack-in-card integration is intertwined with the vertical-card HA card.
Tomorrow I’ll reinstall yesterday’s backup and see if the problem was there as well.
In the mean time, @Ildar_Gabdullin, can you have a look at my other post?