Issue with exporting & YouTube thumbnail

I’m having trouble exporting my project. The main issue is that the YouTube video thumbnail won’t show up. I’ve also tried setting a custom thumbnail image, but the same problem occurs.

When exporting, I get an error message saying there’s a problem with my connection or that my disk might be full. However: My disk has plenty of free space and my internet connection is working fine.

What could be causing this issue, and how can I fix it?