This can result when the faculty member forgets to select the class participants he/she is wanting to email. It's an easy step to overlook. If you want to select all members of the class, there is a check box in the header atop of the class list that, when checked, will select all members. If you wish to email specific students, you can turn on the email check box in their row on the class list. At the bottom of the list, you will see the "Email Selected Students" that takes you to the next step in the email process.
On this next page, if you do not see any email addresses in the recipients list, you know you have overlooked the selection step.
If you receive the "Internal Server Error" message, look to see if your recipient list is empty. If so, simply return to the class list and select your desired audience for the communication.