read more tag inactive

I am actually using the “Wilson” theme by Anders Borén on my www.remarpro.com-blog.
I was able to use the “read more” tag (which is supposed to be a core feature of WP) to get a link after my snippet, directing the reader to the full post.
It just worked fine until recently – but for some reason inexplicable to me – the “read more” tag seems suddenly unavailable. I didn’t change any settings. To be more specific it is not possible to add a read more option, by clicking on the inserter icon, of which the read more option is showing up faintly visible (inactive). I hope someone can help me out here, thanks!

The page I need help with: [log in to see the link]