I've changed my keybindings over and over again to make quickspell work, tried it on the default settings, too.
It will never "save" the spell and when I press the "cast ready" button, my character just does a regular attack (tested in combat).

Does anyone have a solution for this? It gets a bit annoying when you're playing for ten hours and you have to open your spellbook for every single cast.

odd, i dont recall having that problem
well 1st thing to try is to make sure you are running the game as admin by going to where you have it installed to
and right clicking the "Launch Might and Magic 7 - For Blood and Honor" file
select properties and in the compatibility tab there's a checkbox for "run program as an administrator"

with luck this will fix your problem
at any rate its always something to try on any game that has problems functioning

edit:
i have reinstalled the game and checked it out
its working fine for me without the run as admin
no mods just a clean install on windows 8

I had a similar problem. It said to "click here to set spell on a bar", but you actually use the set spell button in the lower right hand corner. Then s does that spell. Hopefully you are just confused by the obvious wording as I was....

I have same issue :(

1) open your spellbook
2) click the spell you want to set as quick spell ONCE
3) click the "SET SPELL" button at the bottom right of the screen

That's it.

Right click on your character portrait, or look into the quick ref. chart to confirm that the quick spell has been set.

If you want to remove a quick spell, simply click the set spell button again.