Hi Arnoud, the license is already activated for arnoudvandalen.nl in my license back-end. As you did a renewal, you should in most cases not have to do anything else in the plugin settings. If you want I can deactivate it for you, so you can re-activate it.
Your license was activated on a subdirectory site. You may have moved the site, which is why the “activated” was still showing, but it was activated for the subdomain. This is the reason you couldn’t activate it again.
I have now deactivated this domain, you should now be able to reactivate it again.
I bought the license for one site, but I am using two different WP themes on my site. Do I have to buy the multiple site license. All under the same domain name. One is the WP website and the second theme is my WP blog.