The main issue that I’m seeing is that the tool isn’t loading the majority of your CSS and JS.
If you click on “View Details” next to “Page loading issues”, it lists each on it’s not loading.
Unfortunately, they’re giving us the “Other error” reasoning, which isn’t helpful at all.
I did some reading, and found this: https://webmasters.stackexchange.com/a/109387
I wonder if it would help to install a plugin like Autoptimize, which does a great job of combining CSS and JS into only a couple of files. Perhaps this would allow the tester to fully render the page as a regular user does.