When creating Active Directory passwords that will be used to login to Dynamics SL Web Apps avoid using the HTML markup characters: less than, greater than, ampersand, hash tag (#), and single or double quotes. These can cause an error message that is not logged anywhere. The message is “Sorry, an error occurred while processing your request.”
The only way to detect this is specifically ask about the password he or she uses to login. The password will work to login to other things, like Office 365 or a Windows workstation.