I had a game refunded because of a DirectX 12 Error that caused the game not to start. That glitch has been fixed. Am I allowed to buy the game again this time ?
I'm asking, because I know that may sound crazy for some of the GOG-Team, thinking I can't make my mind up or something …
any thoughts?

Sure. There's no reason for you not be able to.

And as for your concern regarding them thinking you don't make your mind, no one's gonna think that. It's totally legit to refund something that doesn't work and buy it again when it's fixed. And if for some reason it's still not working after the fix, you can surely refund it again.

I'd have just waited for it to be fixed.

I would try to make sure that the game has really been fixed, before I buy anew.
Or that it was really related to that error, not something on my side.

I did exactly that for Beyond a Steel Sky. Gave it back because of incompatibility, then bought it again once my new computer arrived. Don't worry, nobody's going to scold you for that. ;)

It could raise some questions if the game still didn't work and you asked for a second refund, in which case you'd explain what you have told us here. But no one is going to manually check your purchases and judge your habits. ;)

Works without problems. Every transaction has a unique ID.

I did this twice already - in a way.
Refunded two games, then bought the Deluxe bundle containing the same game plus some other stuff.

I can also confirm this isn't an issue. In my case, it was with Might and Magic 9 that I refunded due to unfixable compatibility issues then repurchased after it went on sale again when I got a new computer.

If the game was refunded fine, and if you want to buy it again fine.

I'd just have some annoyances when No Man's Sky came out, and someone was complaining about the state of the game and wanted a refund, but intended to purchase it again next week after some hotfix was out or something.

If you didn't know the fixes would be coming, getting a refund was the right response. But if the game has issues but they are working on it, just waiting until the update is the better option i'd think, makes less work for everyone all things considered.

However... if you demand another refund after you buy the game because some other minor DX12 or other issue came up, i'd just say waiting and raising the issue, maybe working with the devs with logs to determine where the bug is. Though it could also be a Win 10/11 thing with how often they do patches and break compatibility.