Web admin: Can't add a distribution list

[Alvin]

Hello,

I can't add a certain distribution list to the system.
Distribution lists -> Create New List
List Name: protocol at example.com
Members: ...

When I press 'submit', this error occurs:
   LDAP Error: Could not add object  
cn=protocol at example.com,dc=example,dc=com: Type or value exists
When I press 'submit' again (scientific method: does this happen every  
time?)
   LDAP Error: could not rename to  
cn=protocol at example.com,dc=example,dc=com: Server is unwilling to perform

There is no currently existing 'protocol at exameple.com'.
Is the name 'protocol' somehow reserved?