Alright, so this is probably not going to work for everyone, but after several days of frustration and trying many, many strange things, I finally have made a spore account.

Heads up, this method will require you to spend an extra little bit of money. (I paid around 2USD). I was getting quite frustrated and desperate, so I did pay.

Here's what worked for me:

1. I contacted EA Support through their support live chat, asked them to add Spore to my Origin library (you will need to provide a proof of purchase, here's a helpful link describing what you need exactly: https://help.ea.com/in/help/account/how-to-provide-proof-of-purchase/)
2. Once you have it in your Origin library, install Origin and try registering in-game. If you get an "unknown error", proceed with the next step.
3. Buy Spore Creature Creator on Origin (https://www.origin.com/pol/en-us/store/spore/spore-creature-creator). Once you checkout, in the purchase confirmation page, you should see the following: "Includes: Access to MySpore".
4. Launch Spore and try registering again. It should work now. You do not need to even install Creature Creator for this to work, once it's in your Origin library, you should be able to register.
5. DO NOT REFUND CREATURE CREATOR, CREATURE CREATOR GRANTS YOU ACCESS TO A SPORE ACCOUNT. If you refund it, you will lose access to your account until you purchase it again.

I tried asking EA Support to refund me and leave Creature Creator in my library, since I only needed it to fix an issue in Spore, but had no luck. If you really want your money back, you can try asking too.

The problem seems to be that for you to be able to register, your Origin account needs to have a store item called "MySpore Access", which is NOT included with Spore and IS included with Spore Creature Creator.
It is quite strange that you don't get MySpore access along with Spore, only with Creature Creator, but doesn't seem like EA is going to do anything about it...