Is it important for SEO, with which company a domain name is registered? And more particular - is it okay to have my domain registered here: MinmaxGroup? Or would be I better off transferring it to GoDaddy, for example?
The registrar for a domain does not have any impact on SEO. It has nothing to do with the website, its content, or its relevancy for any given search term
It doesn't matter SEO-wise where you purchase or register a domain.
If you also use the same company for hosting it will have an impact on where the domain is hosted and what IP your domain gets, which of course also might have a performance impact as well. Say you host your site in Togo, you might experience slow speeds.