FMSA 11 661 warnings
I figured this one out I think, but I will post it since I could find nothing on warning 661 besides an old issue with Server 9, and I didn't see the solution I came up with anywhere.
This discussion is specific to FMS or FMSA running with external authentication turned on, and the external authentication is Active Directory.
My problem was that when I turned on email notifications for FM Server warnings, I was getting bombarded with warning 661 emails that looked something like this:
Warning 661 FMSERVERNAME Client "username [i.p address]" authentication failed on database "Database_Name.fp7" using "Admin [fmapp]"
In short, the problem apparently was that I had certain files, like my startup menu file and certain low security solutions, set up to log in automatically with the default "Admin" user with no password. When the user tried to open or reference another file that didn't have that Admin user, but was configured to handle authentication through Active Directory, I would always get the warning e-mail, even though they did authenticate fine and the user received no error or warning.
The solution was to not automatically log in the "Admin" user with no password in these situations. I unchecked the "Log in using" option under File/File Options, and then I created a new account under "File/Manage/Security" just called "Users" with full access privileges. I set this account to be authenticated via Active Directory. Since by default, all Active Directory users are members of the "Users" group, this functions just like before, but it uses Active Directory rather than a local FM user for its automatic authentication. Now when this file attempts to access or open another file, it doesn't try to do so using an FM user that may not exist, but instead respects the authentication order in that file's list of accounts, and I get no warning e-mails about this.
This cleared up my problem. Maybe someone has something to add or correct here, but hopefully this will help the next person searching for this 661 warning message.