There are a lot of People facing this unknown error and sending ticket to support.
The error is “404 Forbidden: Request Forbidden by administrative rules”
I got the same problem and sent a ticket to fiverr but its not the error from fiverr side. its an error with your internet connection.
Try to restart your internet router OR Change the internet connection.
By restarting my internet router its fixed and I am using fiverr as usual.
I hope this information will solved your problems.