There is some sort of issue going on with the server. Do you have access to error logs? It appears the entire server is going down when this happens. I can't access any pages and things start acting very strangely. I'd contact your hosting provider and see if you can get more information about the exact error and why the site is going down.
The image upload accesses hundreds of pages quickly, but it appears to me that accessing any page multiple times will bring the site down. I see it is failing on the line:
so it has something to do with the server session management, but it appears to be an issue your hosting provider will need to address.