Unable to create user

Hi,

Long time user of GTA Service Desk.  When I try to create a particular user I keep receiving the following error page: 
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Oh No! That wasn't meant to happen.

If you need assistance please get in touch with our support team at support@gotoassist.com

Need Help? Email support@gotoassist.com

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This is very frustrating and when I send an email to support@gotoassist.com it bounces back.  I am able to create other users ok.