Admin dashboard not displaying properly

Hello, since installing this plugin, my admin dashboard doesn’t display correctly, and I’m struggling to find the solution. I read that it is to do with CSS not loading properly?

Is there a setting, or a set of instructions, to follow to solve this issue? I have followed the instruction to use ‘/’ as a parameter to make all pages secure on my website.

Thank you in advance, this is quite worrying for me!

Here is a picture of the issue