Bug with Virtue Mart + VM Invoice (Error 500)

I have a bug in VirtueMart with a module call VM Invoice which generate a Error 500. VM Invoice generate à PDF Invoice sent by email. While we send the order, VirtueMart send the order (the email is received), but the website show a Error 500 (will provide log file to winner), if I refresh, then the error disappear and everything work fine, but we must get rid of this error.