Hello @Luinenburg,
I have successfully logged in to your site. It appears that you are using an older version (4.1.0) of the theme that is no longer able to connect to the Themeco server. I would recommend that you manually update your theme first. You can download the latest theme from your dashboard, https://theme.co/apex/dashboard.
To know more how you can manually update the theme, please check out “Manual Updates” section in this article:
Once you have updated it, you may now use the license code to validate the latest X theme.
Hope this helps. Kindly let us know.