I went back and edited the 5th entry on my Accounts list using the data for the 6th entry that I was trying unsuccessfully to add, and it worked fine ... the User Name was not rejected.
So it is acting like the limit of accounts is a total of 5?
The manual section 1.8 says The maximum number of accounts/radios is limited by the computing resources of RSS. Tests have shown this limit to be 4 or 5 accounts/radios active at one time." I suppose this is my problem, but I would have thought that "active" meant "connected". Simply listing should not take resources.