I'm having a similar issue. Sandbox user creation failed, with the vague error message "An unknown issue has occurred. Contact the website administrator." I followed all constraints listed in the linked document. The only thing I can think of that might be problematic about my inputs is that the email address for the sandbox user is on the mailinator.com domain, which perhaps eBay flags to prevent account creation since Mailinator isn't for "real" email addresses? I'd like to be able to use Mailinator for this if possible, but will make "real" email accounts elsewhere if necessary
