I am on day 9 with my wizard and Erasmus doesn't want to talk about magic, and thus giving me the Whirlwind spell. In fact it was never a topic that I could choose. I did not visit Erasmus in the first few days, so now I am wondering if I screwed up and won't be able to get the spell anymore.
Does anyone have experience with this and know a possible solution?

Yeah, you missed your opportunity to get the spell - the moment the thief robs the bank, Erasmus skips to his final conversation state, skipping all of the previous states.

You've gotta repeatedly visit him in order to get the whirlwind spell - I believe it's on the second visit that you can get it. You'll know when Fenrus decides to take you on a tour of the house.

for what it's worth, I never found the spell itself all that great, but it's a part of 100% completion for the Wizard so, unfortunately, that sucks.

Thanks for the answer, I guess I will complete it without the spell then. Just kinda wierd they didn't give the spell during the first conversation you had with him at the start of the game.

The reason is actually rather silly.

See, the conversation state that allows you to learn the spell is the same one where Fenrus teases him about starting a fire, and Erasmus responds that he simply put it out with a Whirlwind spell.

Which is probably what's meant to prompt the hero to even ask how to LEARN the spell in the first place.

Like I said, though, it's a largely pointless spell IMO, and I almost never use it. Hide + Dazzle + Frostbite is an overpowered set of spells in this game, with the result being that enemies will stand there dumbly as they get chilled to death.

Whirlwind's effect is to spin an enemy in place while you deal damage over time, but it doesn't last long enough to actually be worth the MP cost - though I must admit I've had a lot of fun stacking Whirlwind spells on top of an enemy and make them spin so fast they're horizontal before they snap back to a vertical position.