You need to clarify or fix the sync between the store site and the forums

[Dash,Polder]

If you register with the forums first it doesn't mean the store can handle the same log on credentials, it will not accept a name over 15 characters while the forum does.  The store site retains your old data base and if you use the same email there will be a disparity and you can't log in to the store, you'll have to use the change password option to your mail to gain access. 

If you used characters from your user name on the forum that work perfectly fine, they don't necessarily work on the store site which has harsher restrictions.  If you screw it up you can only change your name once in a 30 day period, so I can't make a purchase for 30 days now.

You need to seriously think about syncing up and using the same set of rules for the forum and store.     

[Dash,Polder]

You can buy it, just going to be stuck with two separate and distinct logins, so a bit confusing. 

[AlexandreCosta]

I believe they separated the forum and the game acconts for security reasons.

[FuriousMeow]

The forum is a new login. The store is the same old login. Not really all that confusing.