Confusing and annoying for beginner,

Had to register twice when first going to this link https://mikrotik.com/client/ even mentioning in the account page you can address a "forum username" which I did but than it won't login on the mikrotik forum using username nor emailaddress.

The second time was register directly on this forum page although using the same email address and username it does work this time, since I'm writing this post.

Maybe both accounts can be merged?

No they can't. Forum is a separate entity for user to user discussions, not related to mikrotik.com account.

Does this mean there is another forum connected to the mikrotik.com account since it mentions forum username in the account page?

No, that field is purely informative and has no function.