Question: Do i have to resync usergroups on every new member? I see if i do that and then delete user cache it is works ok.

Rune

No, you don't need to do that every time. We've never faced such a strange issue. And I still think this is a 3rd party plugin conflict. We'll test the wpForo registration form, but it has all WP hooks and can be easily affected by other plugins.

Also I'd like to see a screenshot of Forums > Usergroups admin page. What is the default Usergroup you set for user registration?

As far as i now the only plugins i am using that interfere with the registration process is SEO by Yoast and WooCommerce Smart Coupons.

I have tested with differen usergroups as standard but with no luck. Now using WC customers as standard.

Just updated my staging site with all the same plugins and the wierd thing is that here it works just fine, also with the popup registration.

So this is really wierd and i think i have to dig deeper into it.