Can't log in

My colleague received a file sharing invitation via Box from one of our business partners. After receiving the invitation email, he gets an error screen when he tries to log in. He needs to access these files urgently, so please advise how to resolve this issue. His email address is <personal-information redacted>. He has reset his password twice, but this hasn't resolved the problem. After clicking the "Accept Invitation" button in the invitation email and signing in, he gets an HTTP 500 error and then the screen goes completely white.