[Reported] Cannot cast spell / no target available

PC, steam
Game mode: Events - Daily siege (1 day), timer mode

I don’t know why but I was unable to cast the spell Tentacle Slap against 2nd opponent in this turn. Error message as shown on screenshot: no target available

Happened again in the 2nd battle.
On first turn I casted Dark Ice, Channel Darkess and Dark Firebolt (killed the first opponent).
On second turn Tentacle Slap cannot be casted

I don’t have a screenshot, but it happened to me as well when I tried to cast the ultimate spell. On the next turn everything was okay.

Same for me

The same error message has occurred to me also. But what I have noticed is that if there are TWO spells filled at that point, as I have tested, one can cast either spell, no error message.