An HTTP 500 error is a general error message issued by the server when no more specific 5xx message is appropriate. You will need to get the log(s) from the failing server to determine what exactly is going sour.
in reply to HTTP error response code 500 using LWP::UserAgent on one site, but not on any other
As a work around, you can have LWP send a user agent string identifying your 'browser' and one of the browsers that the remote server accepts. Note: this may lead to other "compatibility" issues.
I Go Back to Sleep, Now.