I just tried to reset the password of one of our users. I entered the password and confirmed it, and selected "Send Login Info" and saw a 500 error from the server.
I tried again and did not select "Send Login Info" and it worked fine.
A note that this user's email address is wonky; when you send emails to it it works but you _also_ get an error saying it can't be delivered to a hotmail address. His email isn't hotmail, it's weird I know, I don't know why (probably some mail forwarding thing from ancient times) and I don't know if it's at all related but I wanted to note that just in case it is.
Please submit a help ticket with the email address in question and the exact text of the error message you're seeing.
I just tried the same thing with my account/email and saw no error. I've asked the other user for permission to change his password again and if it's OK I'll retry with "Send Login Info" and submit a bug report with the 500 error text if it happens again.